Actually, Black Templars aren't completely out(Partially because of what I can only assume was Miguel getting in a fist fight with a GW overseer.) but it's going to be very late if at all. World Eaters, Emperors Children, Death Guard, and Thousand Sons seemed to be part of a bundled 'God Dedication' expansion pack after launch. I'd like Emperors Children sooner, but I can wait.
Gotta go with Dark Eldar. Their "power through pain" ability could give them a buff as they deal damage to the enemy whi le at the same time debuffing them with different poisons. Here's a little write-up I did for a similar thread in the Founder's Lounge: -=Dark Eldar=- (artist) "Why do we ride atop these elegant craft? The better to hear the screams of our prey as we ride them down, to savor the fear etched on their faces, to taste the tantalizing tang of their blood in the air as an appetizer before the feast. But most of all we ride them so that the slaughter may begin as soon as possible." ~Dariaq Bladetongue of the Pierced Eye Sub-Factions: Kabal of the Black Heart Kabal of the Poisoned Tongue Kabal of the Wraithkind Kabal of the Baleful Gaze (Kabal images from Lexicanum) Classes: Kabalite Warrior (Standard) Kabalite Trueborn (Heavy Weapons) Wych (Assault Melee) Scourge (Jump Ranged) Hellion (Harassment Range/Melee) Elite: Wracks Mandrake Grotesque Hekatrix Bloodbride Heroes: Archon Haemonculus Succubus
When one says the Dark Eldar are dwindling in number, what does that mean? I mean, we're talking a big universe here, humans being the most numerous race. Numbers in the millions mean nothing to most of these races. And even if it does, millions is still millions... Eldar themselves are considered a dying race, and they're being implemented and keep throwing themselves out there. Dark Eldar, although probably of even less number, still can wage war. And if Arkhona is as important as it is to bring so many races to it at once (four races is already pretty darn good.) I'm sure even dying and dwindling races would be brought into the fray.
millions? a single IG trash task force is millions. the entire humanity? we don't have numbers for that.
More factions! More Factions maybe add some soul harvester element to their executions! "The Dark Eldar are kindred to the Eldar, an ancient and advanced race of elf-like humanoids. Their armies usually have the advantages of speed and technology, though they are often lacking in resilience and numbers" http://wh40k.lexicanum.com/wiki/Dark_Eldar "Their armies, like their Eldar counterparts, usually have the advantages of mobility and advanced technology, though they are often lacking in resilience and numbers. " http://warhammer40k.wikia.com/wiki/Dark_Eldar Still i do agree, lacking in numbers can be in the billions for all we know.
Not to say they won't get in, they probably will, but the Dark Eldar are even more doomed then most races. The Eldar race is indeed counted in millions(Except for in that one Matt Ward supplement we don't talk about.), rather then the trillions of members of humanity and so they have to be careful with what resources they devote. But the Dark Eldar also have the added bonus of a small Eye of Terror about to explode through Khaines Gate any day now and being plunged into a civil war by this point in time as a result.
Dark Eldar aren't really "dwindling". They have mastered creating test tube babies called Vatborn. They are fully Eldar and possess souls. The reason why the Dark Eldar don't just take over the universe (a bit of hyperbole never hurt anyone) is because they are extremely fractious and the constant in fighting keeps them in check. Kinda like the old saying, "God created alcohol to keep the Irish from taking over the world."
So if their scale from screwed to not a chance is off the charts, what do they have to lose by going to Arkhona? And what would their goals be anyway if all they do is prolong the inevitable?