Oh, I know! I'm totally okay with that. Don't think I'm anti-'old stuff'. I'm just saying that, with a lot of stuff, things aren't *really* set in stone that much. For example, the 13 regenerations thing we've gone over in this thread a few times before. Yes, it's there and has been mentioned in old-Who as a 'rule'. But when push comes to shove, they'll figure out a way to get around that relatively easy.
And that's what I mean with the other old stuff, too. It should remain standing, and it's all important and good, but don't think for a second that a relatively obscure character like the Valeyard (at least to today's viewers) will still be in the same 'state' he was all that time ago (non-existent due to the Doctor's actions, as you say) if they wish to bring him back. The concept behind the Valeyard would still be there, and they'll be able to expand on all that stuff, but the writers probably won't feel *that* restricted by the rules set out by other writers all that time ago. They'll figure out a way around that. Which is good, because story beats almost all.
Not to make this post too long, but I think a decent example of that is the Master. He lost all his regenerations in the old-Who, right? By the time the TV-Movie rolled around, he'd lost them all again and died... again. And then the John Simm-Master was yet another last-ever-Master, but even that particular Master got a second chance in The End of Time. That's what the Who-writers seem to have done almost always: yes, things happen and characters die or never exist. But if they love the concept, like RTD loved the idea of the Master, they'll bring that concept back and nudge around the continuity a bit.
That's probably partially because there's not an entirely proper Doctor Who Continuity. If you take the TV-show, the spin-offs, the books, the audio-stories, the video-games, all that other stuff into account, sure, it ties together, but very difficultly. Even if you take just the TV-show, there's still almost 50 years of TV into account, and within that there's a whole lot of contradictions.
I just don't think people should get too hung up on there being a neat and proper Doctor Who continuity, with a nicely working timeline that has everything plotted out straight. Most writers on the show now or in the past didn't really seem to care about that *that* much too. They'll create new concepts to work with on the show, as well as reusing old concept as they see fit. If they can fit it in with continuity, fine. If they just want to use The Master again even though he's been really, really, really dead half a dozen times now, that's still fine.
I'm setting a pretty heavy schedule!