I don't think Han is coming back. The guy is 73 years old, doesn't like Star Wars much, and I doubt he'll need the paycheck.
Episode VIII is scheduled for 2017, Episode IX is scheduled for 2019 and I don't think they already filmed everything
It would be a huge risk to keep Harrison Ford on board just to please old fans of the series. IMO the main reason we saw so much of him in Episode VII (and so little of the other 4 (Leia, Luke, C3PO & R2D2) was because they planned to kill his character off and make this movie a sort of last goodbye to the fans of his character Han Solo.
I half expected Chewie to die too (and the falcon in a blaze of glory), but am glad he didnt. That character is also much easier to replace with another actor if the need arises (sorry Peter). The character Chewie doesn't look that old. With an average lifespan of 400 years, Chewie would be around 230 now, so still many years ahead of him.
We love the old characters, but Disney wants to attract a young audience for years to come, hence the young newcomers. We've got 5 new main characters (Rey, Finn, Poe, BB8, Ren) and some new returning side characters (Hux, Phasma?) plus the old group. That's simply too many characters to keep track of in a 2,5 hours movie and give each of them some credit.
We barely saw Leia, Luke, C3PO & R2D2 and they were just in episode VII for linking the new trilogy to the old and attract the old crowd who want to see their former heroes once more on the big screen. I do expect to see much more of those 4 in episode VIII and I see most room for Luke & Leia. Unfortunately R2D2 has been made rather obsolete with the introduction of BB-8 (2 funny astromech droids is a crowd (remember Chopper vs the imperial astromech droid)), so I fear his (and C3PO's) role will be rather small. I think it's a bit of a shame because C3PO and R2D2 have been a very dominating pair in the other 6 movies.
So I expect more old-time characters to die in future movies just to make room for the new ones.