i didn't see any rules or regulations on this one, so i guess it's just open to all kinds of impossible reunions. but i'll try to keep it at least semi-realistic.
Hardy Boyz: Team X-Treme has always been awesome and i loved their ladder matches. plus, it's a pretty good point to say that without Jeff, Matt very often gets overlooked. right or wrong, just seems to be the way. so either Matt opens the door for Jeff to get back in WWE or Matt goes to TNA and then we could have a Hardyz vs. Team 3D reunion, and those matches would be good to see again.
E & C: of course! who doesn't wanna see this team reunite? the only thing better would be a feud. either team up and bring mega credibility back to the tag division and then feud later, or feud now and tag later. either way, i wanna see both happen. but if they team now for the tag division cred angle, then, as stated earlier, they could feud with Miz using the whole "awesome vs. reeks of awesomeness" thing and that would be totally and epically hilarious!
Morrison: either with Mercury for MNM (especially once Melina heals up... it'd be a good fit with the other two teams that have a female awesome valet; Hart Dynasty and Uso's) or with Miz. either way. Morrison needs to go heel and go back into the tag division. that was his best stuff. plus, if and when the team splits, he can then feud with the partner and maybe for the midcard titles before trying again for the main event.
Kane & Big Show or Big Show & Undertaker or Kane & Undertaker: i've loved all three of these pairings. probably my favorite was just the Brothers of Destruction. they were so unbeatable! plus the whole [kayfabe] family thing made it that much more awesome. but any pairing of the monsters would be cool. i'm still crossing my fingers for a Big Show & Great Khali team. at least it might make Khali not so terrible. might.
and lastly, NOT Legacy. i thought they were awesome and i actually enjoyed their feud and matches with DX. but i'm super looking forward to The Fortunate Sons stable and team with the Dibiase Brothers and Joe Hennig. plus that leaves the door open for Cody Rhodes and Goldust to team up, and i think that would be awesome. so this wouldn't be a reunion, per se, but still thought i'd throw it in there that i don't wanna see Legacy reunite if only because we could get two sweet tag teams instead of one if they don't.