The Stalker is certainly a very specialized Archetype, being much more like a Rogue class for another MMORPG than its hero counterpart the Scrapper is. The Stalker is even more specialized that its villainous partner the Brute. Still, there is room for a number of different subcategories within the common concept of the assassin, striking a death blow from the shadows. Not all Stalkers are alike, and they do not all play alike.
Ninja:
Primary: Martial Arts, Ninja Blade
Secondary: Ninjitsu
Pool Powers: Leaping
Possible Origins: Natural, Technology, Magic
Examples: Elektra, Curare (Batman Beyond)
The concept of the Ninja is pretty obviously the inspiration for the Stalker. It is similar to my concept of the Stealth Scrapper, on the hero side. All Stalkers, however, rely on the tactics of the Stealth Scrapper, they sacrifice defense for the innate ability to sneak around, divide up their foes, and strike them down one at a time. They don't need the greater defense, since they do take on foes one at a time, but this makes taking on groups more difficult.
The Ninjitsu Secondary adds a number of interesting control-type powers to help you deal with large groups. It is the only Stalker Defense set that includes a secondary Placate, and an AoE Placate at that. Its direct defense is somewhat less than that of the other Secondaries, but Stalkers rely less on defense anyway. And the control powers make it very unique, and more active than other sets. I commented in the description of the Stealth Scrapper, that I felt it would be interesting if they had devices to snare their enemies, and it looks like that is exactly what the Ninjitsu devices are intended to do.
Agile Totem Animal:
Primary:Claws, Spines, Martial Arts
Secondary: Super Reflexes
Pool Powers: Leaping, Speed, Fitness
Possible Origins: Mutant, Magic, Natural
Examples: Catwoman, Carnage
Although the power of the Agile Totem Animal sometimes comes from an animal of some sort, it is also possible for this Stalker to be a martial artist. However, unlike the Ninja, which uses devices and traps to supplement its defense, this Stalker relies only on his superhuman agility and reflexes. Thus, and due to the similarity between this concept and the Scrapper version, I am using the name that refers to a totem animal.
Super Reflexes provides considerably more Defense, that is, dodging ability than Ninjitsu, but it does not have the utility or flexibility. Without the ability to heal, even a single hit can force a Stalker to retreat, and with Super Reflexes, that hit can come at any time, unexpectedly. Thus, the Agile Animal must use movement to keep out of combat, being exposed to melee attacks for as short a time as possible. Ranged attacks can be somewhat better to deal with, although it can take a while for the AoE defenses to get up to strength, they are much stronger than the Ninjitsu version.
Healing Totem Animal:
Primary: Claws, Martial Arts, Spines
Secondary: Regeneration, Willpower
Pool Powers: Leaping, Tough
Possible Origins: Mutant, Magic, Natural
Examples: Werewolves (sort of), Deathstroke/Slade
As with the Regeneration Scrapper, this subarchetype is very well known from the comic books. Healing villains are usually more resiliant in nature, however, and thus might be better implemented as a Brute. The only problem is that Brutes cannot (currently) have Regeneration. Thus you must either settle for much less resiliance and rely on your assassin strike to turn the tide, or build your healing beast as a Claws/Willpower Brute. There are really two versions of this subarchetype, the big hulking types like Wolverine's foe Sabretooth would be the Brute's Rampaging Beast, while this subtype is more the stealthy, cunning, but less monstrous and more human type.
Regeneration is very weak as a Stalker, because hit points are somewhat lower than a Scrapper, and thus Regen Stalkers heal slower proportionally. Ironically, though, Regeneration is more consistent than the Defense-based Secondaries, because there is no risk of the sudden, surprising lucky hit that forces you to retreat. In essense, as long as you keep the number of foes down low enough that you can survive the initial counterattack, you can keep up the strategy, going from target to target, indefinately.
Willpower is a somewhat stronger Secondary, as it provides both a heal (in the form of Reconstruction, which is unique to this version of Willpower) and a combination of Defense and Resistance based powers. This results in a somewhat lower reliance on the Stalker's hit points, while still reducing the "lucky hit" factor of a purely Defense-based set. Still, recent changes to Stalkers have brought their hit points much closer to that of Scrappers, improving the performance of Regeneration, and Reconstruction really isn't as effective as Rise to the Challenge in large groups. So while Regeneration still has a reputation as a poor performer, I'm not sure it's still justified.
Mystic Assassin:
Primary: Dual Blades, Ninja Blade, Broad Sword
Secondary: Super Reflexes, Willpower
Pool Powers: Teleportation, Speed, Medicine
Possible Origins: Magic, Natural, Science
Examples: Morgiana (Ali Baba's slave girl, who kills the 40 thieves)
While a Ninja is a mystic assassin, this subarchetype is based upon the original Arabic "Assassin", or any mystical, somewhat exotic assassin of the more Western ideal. Usually this type of assassin will use one or more knives or blades, often poisoned, instead of relying on martial skill or animalistic fury. I add in the Power Pool Medicine to reflect how this subtype might use drugs or poisons, which could help allies as well as being dangerous to their foes. Of course, their mystical connection or even actual magical powers could make them able to appear out of nowhere, or move with lightning speed.
The associated Secondary could be either Super Reflexes, for a more agile, active style, or Willpower. This could be considered a subtype of Agile Animal or Healing Animal, and in fact the Stalker may take an animal name to inspire him. (Or intimidate his foes) The use of weapons specifically separates out this type, though, and set the style for the character. Ninja Blade is somewhat faster as a weapon, Broad Sword is slower, and Dual Blades has its interesting "combo" feature. These sets are slightly different for Stalkers than for Scrappers, because of the need for an Assassin Strike, and the lack of an AoE.
Tech Stalker:
Primary: Energy Melee, Dual Blades, Ninja Blade, Broad Sword
Secondary: Energy Aura
Pool Powers: Teleportation
Possible Origins: Technology, Science, Mutant
Examples: Warp (Teen Titans, sort of)
The Tech Stalker usually is best explained as a high tech villain, wearing a costume made of some sort of cloth that can absorb and manipulate energy. Of course, the villain could also be a mutant or the victim of some science experiment, with the ability to use energy. The Energy Stalker or the Power Stalker, though has as his primary ability to power to wrap his body and his fists in energy, to use as defense and a weapon. He can even control light itself, bending it around his body to make himself invisible.
Like Super Reflexes, Energy Aura is a Defense-based set, and so the Tech Stalker is vulnerable to unexpected hits which cause large amounts of damage. Unlike Super Reflexes, Energy Aura does have a heal. However, it is fairly weak, and depends on the number of foes in range. It does have a number of other utility powers that can help the Stalker get out of trouble, including Repel, which is an AoE knockback, useful for clearing the way for a rapid retreat. It is also, as with the Brute version, very Endurance efficient, although Stalkers usually don't remain in combat long enough to have Endurance issues.
Dark Stalker:
Primary: Dark Melee, Dual Blades, Ninja Blade, Broad Sword
Secondary: Dark Armor
Pool Powers: Teleport
Possible Origins: Magic, Science, Technology
Examples: Vampires (sort of), Scar (Full Metal Alchemist)
Dark is similar to Energy in many ways. Dark is Negative Energy, so it could be considered the opposite to Energy. Energy could also be considered Light, the counterpart to Dark. In any case, the Dark Stalker draws upon the powers of darkness to hide himself and stalk his victims, and Dark, as you would expect, is very good at that.
In strategy, the Dark Stalker is similar to the Dark Scrapper or the Dark Brute. Dark provides a great deal of regeneration, as well as both Resistance and the ability to avoid attacks, both as Defense and in the -Acc on its attacks. Thus, it has a large toolbox of defenses, something unusual for a Stalker. It has neither the vulnerability to alpha strike that Regeneration has, nor the risk of suddenly being forced to retreat, like Super Reflexes. This makes Dark possibly the best defense for a Stalker.
The only drawback to Dark is its high Endurance cost, and the likelihood of AoE effects like Dark Regeneration and Cloak of Fear drawing aggro. While Fear is useful for keeping foes from attacking, similar to the control powers in Ninjitsu, it also requires alerting a foe to your presence. So it can be somewhat of a double edged sword.
Electric Stalker:
Primary: Electrical Melee
Secondary: Electric Armor, Ninjitsu
Pool Powers: Teleport, Super Speed, possibly none
Possible Origins: Technology, Mutant, Science
Examples: Unknown
While it may seem odd for a Stalker, which relies on stealth and being hidden, to use the bright energy of Electricity as a power, in fact this is no different from Energy Aura. The source of the electrical power that "hides" the Stalker could be a high tech suit, only with an electrical field. One drawback of Electric Armor is that it is resistance based in nature, which means it cannot avoid attacks that would otherwise break Hide, as Energy Aura and Ninjitsu can. And its only heal, Energize, is very weak compared to the other resistance-based set, Dark Armor.
Electrical Melee, by contrast, is usually considered to be a good choice for a Primary. Along with Energy and Dark, this is one of the few choices for a Stalker that is not purely Smashing or Lethal damage. And Thunderstrike and Lightning Rod are strong AoE attacks, something unusual for a Stalker. As with Dark Armor's defenses, all this AoE can draw more aggro than the Stalker can handle, making it a bit of a double edged sword. Still, the ability to use Lightning Rod's ability to teleport into a group, even though it does not Critical, can be quite satisfying. Ninjitsu makes a good combination with this not just for the ability to link it to a Technology origin, but to use Smoke Flash after the Lightning Rod to Placate the group just aggroed.
Stealthy Speedster:
Primary: Martial Arts, Electrical Melee
Secondary: Super Reflexes, Electric Armor
Pool Powers: Super Speed
Possible Origins: Mutant, Science, Technology
Examples: Quicksilver (sort of)
While the concept of the Speedster is more of a generic type, and thus fits the Brute, the generic melee archetype on the villain side, there are certain connections between Super Speed and stealth. Super Speed has stealth as a secondary effect, at least in PvE, so it stacks well with Hide. And a Speeder can use his super speed to sneak up on a victim and make an assassin strike -- IF he is an assassin.
Thus, the Speeder has two villain versions, one a Brute, the other a Stalker. It is up to you, the player, to decide which better fits your concept. Is your Speeder a constant fighter, continually zipping from foe to foe, without stopping, or does he strike once, and then speed away? If the latter, then a Stalker is probably the better choice of the two. Although it can take some time to set up an Assassin Strike, you can think of that as your Speeder being able to think much faster than his foes, and having plenty of time to set up his attack when only a second passes in "real time".