Needs to be a FPS, that third person view is crap. The online co op and multiplayer would be so much better in FPS. That would be my wish...
each to their own....i quite like the 3rd person view. The sensible thing to do would be to make a toggle and let the user decide.
I'd like to see SM2, but now that the MMO is scrapped, what i'd really like to see is a dragonage:origins style RPG.
Base it on deathwatch and you could have a good range of party members to choose from, with different fighting styles, abilities and personalities. You could include guardsmen and Inquisitors, but even going purely on marines you have libbys, chaplains, techmarines, kill-marines, black shields, devs and assault marines. Mixing in their chapter traits, you could have a really interesting choice of party members.
Instead of loot, you can earn honour to buy blessings, purity seals, armour upgrades and weapons. There's a history of Ordo Xenos using alien tech (with some deliberation) so this could form the basis of some epic quest rewards.
For the main character you'd choose between 1 of several chapters, which defines your traits and backstory. The first part of the game would play out within your own chapter, acting as a tutorial and advancing the story to the point when you're recruited by the deathwatch.
From the pen & paper we already know how well it works as a setting for an rpg, the missions and storylines practically write themselves. Throw in lots of blood and epic killing blows, and i think we have a game here