Things Dark Millennium Online must include.
Roleplay Servers - A good way to bring people in from other MMOs, as well as draw Dark Heresy players in. This does come with some extra work, interactive things in the environment, a mechanic for walking, an emote system with animations (which frankly, if you don't have a few of these anyway, you're doing wrong.)
Playable Sisters of Battle - Because girls can't be Space Marines, and they're my favourite race.
Playable Imperial Guard - Balancing this will be a bitch, but they're a staunch favourite. Who doesn't want to be a Commissar?
Playable everything-else-except Necrons and Tyranids - This might well upset players of these races, but I'm leaving them out because there's absolutely no social aspect to them, Tyranids are a hive mind who skitter around eating everything, and Necrons are long dead beings that don't do much except flay and destroy things. Neither of them are very talkative.
Unique Classes - Every race is different, after all. There are Librarians, Celestians, Stormtroopers, Raptors and more. Take note of this. The least you can do is give them different names, even if Raptors, Assault Marines and Seraphim all do the same thing.
Customisable Armour - Chances are you've already implemented something like this already, but whatever system you're going to use for armour, it's important you realise people want customisation, so include a (conservative) recolour system (to prevent such things as bright pink Imperial Guard) and transmog. Better yet, have an armour system like Space Marine's. Even Imperial Guard will need customisation, not everyone wants to be Cadian.
Don't make the same mistakes Relic does - Namely limiting everything to Blood Ravens, Orks and Eldar and making us pay for the rest.
And with any luck, decent balancing between Order and Destruction. It's not a great system for an MMO in the 41st Millenium, being able to split in three ways would most likely work better (Imperium, Xenos and Chaos) but however you do it, balance the game like its never been balanced before.
