A feud with Christian would be awesome and something I've been pushing for, for quite some time. In fact, I recall being disappointed when Christian returned face and won the Intercontinental Championship because it meant the feud with Punk that they set-up (Punk injured Christian making him miss WrestleMania) never happened. We've only gotten a few matches out of these two as well, so it'd be relatively fresh.
A feud with Shawn Michaels is probably my dream rivalry for Punk right now. Mainly because Shawn is my all-time favorite and Punk is my current favorite, and yet they've never had a single match against each other. Would be incredible I'm sure.
A feud with Stone Cold Steve Austin for reasons that are way too obvious to explain. Continuing with the theme of "too obvious to explain reasons", Chris Benoit too.