I have to admit, you guys are thinking way too big. As I read through this thread, and the excellent suggestions it contains, I cannot help but see the stereotypical game show counter spinning faster and higher.
Normally I don't comment on threads like this but fuck it, I have actually thought about this way more than anyone reasonably should, with a ~10 page concept I made for fun and to test a new piece of writing software (Scrivener is pretty awesome btw.)
So, here is the conclusion I reached. As awesome as a vast, sprawling, multi-race, Eve style MMORPG in the 40k universe would be I don't really think it is possible to make such a game within a reasonable, or even unreasonable, budget. I'd strip the game way down to its bare parts for release.
I would structure the game simply at launch. The game is PvE based, and all players are Space Marines (or, if you REALLY want female characters include the Sisters of Battle). The classes fall into general archetypes, I was thinking something like Tactical, Assault, Devastator, and Tech Marines. Each one has a selection of equipment, sometimes overlapping, that determines their "abilities". Moment to moment game-play would resemble the under appreciated (IMO) "Space Marine" game that came out a couple of years ago. So, a combination of melee and shooter game-play with load outs and limited ability selection within a level.
Progression is achievement based, you don't have levels per say but you can unlock gear, upgrades, and whatever progression elements you have through completing certain objectives or kinds of objectives. So, killing 5 hive tyrants would unlock the relic flamer for example (spit-balling here).
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