I swear, if you can "pilot" a Titan in either the unlikely Space Marine 2, or the MMO, I am going to murder people. A Titan is run by a crew, and the leader of that crew(the Princeps) is physically and mentally linked to the machine. Many of the Princeps in the setting become so irrevocably bonded with the sentience of the Titan, they must live inside it within an immersion tank, or risk catastrophic brain-damage every time they disconnect.
Not to mention that the Titan Legions are a sub-faction of the Adeptus Mechanicus, one of the most insular factions in the setting; the idea that they'd suddenly be cool with thousands of Marines, Guardsmen, or whatever the MMO classes end up being just pottering about on their sacred God Machines and casually ordering them about is ludicrous.
Hell, I know I sound a bit anal-retentive, but why bother getting the rights to an established setting if you're just going to change things around, just make up your own dystopian sci-fantasy future world.