When considering the full breadth of variety in 40k, it seems as though no matter how diligent the devs are about introducing new factions some will be missed or fall off the map. Forces like chaos deamons, grey knights and harlequins are well known and well loved factions in their own right but are usually treated as part of a larger allegiance. Maybe it could be easier to implement them as a "special" subfaction for existing forces somewhere down the line, that has radically different classes but still exists as allied under the main faction. This could include harlequins for the eldar, deamons for chaos, and would even fit well with things like the Covens for DA and the Farsight Enclaves for Tau when those races are released. You'd play them the same as any other subfaction, like the Space Wolves or the Ulthwe, but the class choice would be very different and specialized in the extreme. It'd be a lot easier asset-wise for the devs to release than a full faction and could prevent them from being discarded altogether. For space marines it's kind of up for grabs what the subfaction would be. SoB and grey knights would fit very well, but I'm not sure their fans would be satisfied with a subfaction. Black Templars are different enough that they could probably fit the bill. And for orks... gretchin or feral, maybe. What kind of special subfactions would you like to see?
I figure units like Harlequins will be hero classes added to a faction in the future. I got the impression that while EC might launch with only 4 classes for each of the factions the devs had plans to expand the factions further in the future. Man, a Harlequin would be a lot of fun to play as
They said Chaos Demons will come out as a separate faction, but i honestly agree with you that they probably shouldnt. Even though id have an alt in Demons im sure they are far from any notable popularity. (though i think Michael Chan is a Chaos Demons player, or some other dev since i know one said he is). So wasting time, money and development time on making Demon related structures, spawnpoints, announcers, huds, faction council mechanics (lol what would a council of demons look like, a Bloodthirster punching everyone in the face the moment they want to suggest an action) is really pointless when they can easily be affiliated to their respective allied factions. Granted this works for demons and a few other factions, some are trickier. Also lest we forget, if we have them as a separate faction its another entire main base and part of the continent they would have to own.
There won't be more than 4 factions. They'll be rolled into the existing factions as allies. There's any number of reasons why, so I'm not going to cover it again, but that is what is going to happen.
Dark Eldar will likely be rolled in with Eldar or *possibly* orks. How it will most likely fall out will be: Imperium of Man (all the various Imperial sorts) Eldar/Tau Orks/Dark Eldar Chaos/Necron Orks are probably the hardest to clump in with anyone else, however, the Orks can certainly be bought, or persuaded to fight with people instead of against them ("hey, there's a bigger fight over there, and working together, after we win you can always fight us"), while the DE wouldn't be against manipulating the Orks. In addition, it gives the faction more variety of playstyles and variety in general, which is never a bad thing. Eldar/Tau are a natural compliment, both in general outlook, and again, in potential playstyle. Chaos/Necron for the same reasons. A better pairing would be Tau/necron, Orks/Chaos and Eldar/Dark Eldar, but I can't see them making that change. As presented it would also allow a single expansion or two (say, whichever two were least played would be part of the first expansion, the other two the next expansion), and allow for staggered "free" updates to flesh out the forces entirely, all units represented, etc.
Chaos and Necrons? They are completely anti-eachother. Necrons don't pair well with anyone really, but especially not with Chaos for lore reasons. Didn't the devs say they planned on releasing more factions anyway? I haven't heard anything about an "imperium of man" or other teamups
Honestly im not seeing the logic in Chaos/Necron (no souls) nor Tau/Necron for that matter. Eldar and Dark Eldar despise each other from my understanding so that would be like allying SM and CSM (Deldar causing the downfall of their entire race seems equally as bad as what the CSM did to their race, if not more). I honestly doubt there will be any such joining aside from perhaps demons and some imperials with their respective imperials (and that might be a long stretch considering it might bring about a huge imbalance cause the Imperium is the most popular).
From the FAQ: "At launch, there will be Space Marines, Chaos Space Marines, Orks and Eldar. Tyranids will also be in the game, but only as NPCs. We aim to release the rest of the races from the universe over the lifetime of the game."
The only thing the devs have said is that they would eventually like to put everything in they haven't specified one way or the other. Realistically, having more than '4 sides' just won't work well. Allying will be the best option to make a good game. While I agree that Necron/Chaos isn't the best, DE and Tau are both worse allies for Chaos. Though Tau.. maybe, which would make it Imperium, Orks/Necron, Eldar/Dark Eldar, Chaos/Tau, which does fit better with the current allies charts for TT, but I can't see Tau staying allied with Chaos for the long term.