Personally, the news that Caprica has been greenlit as a series, and is looking to reassemble the Battlestar Galactica writers room, was an early Christmas present - Yes, I remain cynical about the longterm prospects of Caprica as a concept, and yes, Battlestar Galactica has had some very bad writing in its time ("Black Market," anyone?), but I'm very happy to see such a thoughtful collection of writers saved from fates worse than creative death. Or, as Mark Verheiden calls it, Heroes.
That said, the one writers room that I'd really want to see brought back together would be Angel's, especially from the fifth season, with Joss Whedon, Drew Goddard and Ben Edlund (amongst many others); the scripts for that final season balanced humor and drama, complete stories and continuing threads, and the need to allow David Boreanaz to do his schtick almost perfectly, in my opinion, and I'd love to see what they could come up with if they reconvened with their post-Lost, Supernatural and Dollhouse experiences.
(Moviewise, the whole notion of "reunions" tends to happen fairly often; Chris Nolan and Christian Bale on the Batman movies and The Prestige, or in the non-SF world, Scorcese and DeNiro on numerous movies (and then DiCaprio taking over from DeNiro, as Marty works his way both younger and forward through the alphabet).)
Outside of television creative teams, I'm sure the comic world would collectively kill itself to see another Alan Moore/Dave Gibbons collaboration as anticipation for the Watchmen movie builds (They actually reunited for a Spirit short story, post-Watchmen; I'm surprised that's not been reprinted recently, to be honest), and along similar movie-influenced lines, a Frank Miller Daredevil would probably also be met with similar acclaim. Me, I'd much rather see more Grant Morrison Justice League, but I think that ship may have sailed...
I can't work out if creative reunions are the triumph of nostalgia over creativity (The Smiths, for example) or the resolution of some kind of unfinished business (The Blur reunion), and as a result, can't work out if I should feel guilty for wanting to see Alan Martin and Jamie Hewlett team-up to come up with something that has the energy, but not the name, of Tank Girl. Is it really that wrong of me to want to see what all the people behind Doctor Horrible's Sing-Along Blog create something new, instead of Fringe and Dollhouse? Uncertain, I leave the floor open to comments; what reunions would you lot want to see happen, and why?