Here are some stretch goals I like:
2.5 million: Ziets!!! Ideally as a permanent member of the inXile team.
3.0 million: Moar artists!
3.5-4 million: Get a real, established, SciFi writer, one who can actually write decent prose and is not just about ideas, to write the dialogue for one of the NPCs. I suggest Iain Banks or Neal Stephenson or Joe Haldeman. Neal Stephenson may be more likely to agree because he is already messing around with video game stuff.
I disagree that Neal Stephenson would be "more likely to agree because he is already messing around with video game stuff". I would say that that makes him less likely to get involved in another project, now that he already has his own project to work on.
I think that writers with at least some experience writing about transhumanism/posthumanism would fit the bill best. Given that Neal Stephenson is already busy doing his own thing, the rest of the Tier 1 candidates, IMHO, would be (in alphabetical order):
Alastair Reynolds
Charles Stross
Cory Doctorow
David Brin
Greg Bear
Greg Egan
Gregory Benford
Iain M. Banks
Stephen Baxter