I personally want to see some of the original X-men movie start to return since the Days of Future Past story bridges between timelines. Though most of all I want to see Cyclops, Wolverin, Jean Grey and Gambit in the film.
Cyclops. I don't care how (I would love Marsden back), he got screwed in X3 and they introduced Havok in First Class so I think he should come back most of all. Plus the film makers teased perhaps having Cyclops in the films again.
Wolverine. Even though the first three X-men films were arguably Wolverine and the X-men, I still want to see Hugh back in an X-men group film. Not to mention that Days of Future Past features Wolverine in it.
Jean Grey. This one is more or less because I don't think you can have Cyclops back without having Jean back. I'll admit that I'm not sure how either would work, but when you've got a time travel story then it already makes the job easier. I'm sure it can be worked out. Plus I wasn't happy with resurrecting her only to die again (not to mention her killing Cyclops and Wolverine's role in being the one to take her on felt like it should have been Cyclops' role)
Gambit. Ok, this one is just because I've loved this guy since I was a kid (though he did feature in the cartoon version of Days of Future Past) and I thought Kitsch at least made a great Gambit even if Origins wasn't any good itself.
Other than that I'd like to see Rogue, Iceman, Storm, Xavier and Magneto (Stewart and MacKellen's version) return, but Cyclops, Wolverine, Jean and Gambit are my main concerns.