At the very, very, very least I would like to see him between Twelve and Thirteen…which is where he’s supposed to be from…he’s a transitional form of the Doctor, right?
I believe so.
The way I’ve seen the Valeyard described is “somewhere between the Twelfth and final incarnations.” Mostly I’m interested in the fact that he’s basically the embodiment of all that is evil in the Doctor, and apparently that is - canonically - terrifying to the Master.
The Dream Lord was sort of a in-between stage, I’d say, towards getting the Valeyard. HE JUST NEEDS TO SHOW UP. NOW.
See, back when I was watching the show during Six’s run originally, it had been said that The Doctor was getting progressively more evil with each regeneration, or darker at least (hence the Sixth Doctor not being the most pleasant chap). Plus you had the Valeyard, who existed between his Twelfth and Thirteenth regenerations. And then you had The Master…this mysterious evil Time Lord who had run out of regenerations and was somehow connected to The Doctor. At the time I put two and two together and decided that The Master must have originally been The Doctor’s Thirteenth regeneration, and that’s why he was obsessed with extending his life beyond the normal span of thirteen regenerations.
Of course, we now know that to not be true…but that was my headcanon for a long time…that The Doctor and The Master were one and the same.