Lesnar/Sheamus would be a good fight but I don't buy Sheamus as a threat to him at the moment. Maybe Roman Reigns. Fucking Rusev or Cesaro would be awesome but I don't see that happening.
Daniel Bryan is absolutely a possibility and imo one of the most likely to be one of Brock's future opponents. Going off whatever kayfabe is left nowadays, Bryan has been built up over the last year as one of the most credible competitors in WWE (credibility in wrestling =/= size... more like marketability). By all means include the size difference as an aspect of the feud, but if you can put Punk in there with Brock without many people getting put off you can do it with Bryan, as long its fun to watch.
To be honest, I'm expecting the first thing Brock will do when he gets back is announce he's now going for the title. He's just beaten Undertaker's streak after all, might as well capitalize on this once-in-a-lifetime momentum.