ideally?
Pepper
Rhodey
Betty Ross
Gen Ross
Doc Samson
Loki
Jane Foster
would all re-appear and maybe a cameo of Peggy in a flashback for cap
That would be somewhat ideal, but idealism has never mixed well with reality.lol
Be that as it may, I honestly wonder on how they're going to avoid supposedly showing anyone else from Thor's own gallery of characters since unless they just have him appear in a flash of light due to whatever has yet to be revealed; then they'll have to go with their other option of having to show Asgard and it would make sense for someone from his inner circle that we saw in Thor to be next to him before he left there on his own will.
Personally, I think the potential problems of bringing in other major characters from any of the Avengers own films, is that not only may it take up unnecessary time, but it may also put more focus on the Avenger that the guest character is associated with as well, and we certainly don't want any one avenger to get the most spotlight (I think.lol)
Though people that I wouldn't expect to be in this film, by either confirmation that they won't be or by story reasons:
1. Doc Samson (He was technically still Betty's current boyfriend and didn't have any real important role in TIH)
2. Rhodey (Don Cheadle Confirmed that)
3. Peggy Carter (Most likely Dead, though they could use a scene from CA as a past flashback that Steve thinks about)
4. Betty Ross (I would have thought that we'd hear by now if someone like Liv Tyler was set to cameo in this film)
5. Jane Foster (As much as I'd like to see Nat in this film, until said otherwise, she is not confirmed to be in this film)
General Ross, now he'd make sense, but then again, we have yet to find out as to why Tony was even sent to talk to him about the Avenger Initiative