I realize that British fans may now make fun of me, since this post is two days late and they, unlike me, have already seen "The Big Bang". But Moffat has done such a good job in avoiding spoilers for who the mysterious villain is for Series Five, I don't feel like I'm going to overlook a ludicrously obvious candidate. Speculation abounds, but nobody really seemed to know prior to broadcast. (Which is, of course, over in the UK. No spoilers in the comments sections, please!)
So this isn't my guesses as to who it is--it's my desperate hopes for who it isn't.
1. Davros. Please, please, please, please, please don't let it be Davros! It doesn't make any sense, it'd come totally out of left field, and Davros has just been used as the season-ending villain (and that came totally out of left field, too.) Anyone would be better than Davros, anyone at all.
2. The Dream Lord. Or the Valeyard, or Zagreus, or any iteration of the "evil Doctor" theme. I just don't think there's much that's interesting about the idea of a villain who's just "the Doctor, only bad", and I'm hoping we won't see that as the menace behind everything that's happened this year.
3. The Master. And speaking of "the Doctor, only bad", John Simm's Master is too self-consciously manic, too overwritten, and was just used in the last big "season finale" special. I'm ready to give the Master a nice long rest and a bit of a rethink before we see him again.
4. Rassilon. I really liked Timothy Dalton's performance, and it would fit the plan that we've seen so far, but as with the Master, didn't he just show up?
5. Amy/River. "Evil companion" isn't doing much more for me than "evil Doctor", plus I like those characters and want them back next season. (Oh, and "evil Rory" would also be on that list, as amusing as the idea would be.)
There we go--soon enough, we'll see whether Moffat disappointed me!