Many of the themes I was thinking about were already listed above, perhaps with a bit different wording. So I'll just mostly comment on some your suggestions:
Art Critique's fake movie titles are very interesting, but might be difficult to fake, especially if not familiar with the film.
Non-nude, grouping, erotic but not revealing are one of my favorite type of fakes.
Also adding a story by making the fake an advert, adding a caption (or the magazine cover & movie poster, which were used once already) and similar things are nice. Here you need to do more than just paste on the face.
I like the debutantes idea
Also forcing the requesters to have certain kind of source pics could be interesting, although it might get smaller amount of requests. I would like the celeb or normal person+celeb or normal+pornstar themes especially.
And out of the already used themes I would like to have again atleast the gangbang and interracial (and reverse gangbang). And apparently the glasses/nerdy/librarian hasn't even been used yet, I would much like that.