The prisoner in the future might well be Zoom/Zolomon. Both Barry and Wally spent much of the final part of the story wondering "where's Zoom?" and vowing to "find Zoom." I'm guessing Zoom is that individual locked up in future Iron Heights.

Superboy-Prime is a remote possibility, but I'm skeptical that it's him. Anti-Monitor is even less likely, as this individual has a personal beef with the Flashes for "what they did to" him. It's true that Barry's sacrifice in COIE # 8 put a serious damper on Anti-Monitor's plans to take down the positive matter Multiverse, but Anti-Monitor has no personal beef with Wally or any other Flash.

Buried Alien (The Fastest Post Alive!)