I don't think Epic wanted to be bought and although they seem a good fit I suspect they would have been a legal minefield for MS. Consider how many agreements EPIC must have in place to support their engine across multi-platform titles/developers. I doubt MS - as tempting as Epic must look - would want to actually be responsible for providing on-going support for a multiplatform engine.
Bioware I think MS did indeed miss as a great purchase. It's cost MS a signature former console exclusive and a former fairly MS centric third party developer. EA will for sure make sure all future Bioware IP/titles are fully multiplatform.
Remedy seem like a good potential purchase although the relatively muted sales of Alan Wake might have tempered any interest MS had.
TBH though I think it is pretty clear at this point that MS decided to rely on one core exclusive, Halo, coupled with a number of funded exclusives and a lot of purchased timed exclusives to drive the initial growth of the 360, particularly in US.
Now I think MS first part focus will be Kinect and I think they will rely on install base, momentum and third party support more for the supposed 'hardcore' along with more Halo and more timed or funded exclusives.
At this point, I'm not there are that many developers left they could purchase that make sense for them at the moment.
Try to be reasonable... its easier than you think...










