IMO the big men that will return to the Spurs next season for sure are Duncan, Oberto, and Horry. I think that Elson is a question mark. Oberto has replaced him in the starting lineup and is playing very well. Elson has been playing less and less since the playoffs started and he's been horrible in the playoffs so far.
Butler is also a question mark. His work ethic has been questioned by Pop so he's probably on his way out. He's still young and pretty raw. He could turn out to be a nice player, but I'm not sure he will on the Spurs.
Ely is gone IMO. He's not going to play much for the Spurs in the near future and I'm sure he wants more then the league minimum even though he's not worth more then that.
Bonner is a nice player but with Horry still on the team his minutes won't be many. I think the Spurs will pass on re-signing him. I wouldn't mind having him next season but he probably wants over $3 million a year and I'm sure the Spurs won't go that high.
The Spurs will most likely bring Ian Mahinmi over from France and hopefully they'll bring Scola over as well. I've been waiting for two years now to see him play for the Spurs. He has proven himself in Europe and now it's time for him to take the step over the pond. Even if it costs the Spurs most of their mid-level it's worth it IMO.