My number one choice would be Black Sabbath of course.
Been waiting since '99 after their reunion tour when the talks began about a new one, but of course we're still waiting aren't we? I liked "Psycho Man" and "Selling my Soul", the two songs they recorded and release during the reunion, but it just left me with my mouth watering.
Another would be an Ozzy album w/ Jake E. Lee, but that aint gonna happen....
Sabbat (UK Thrash band) was another one that put out 2 really good albums then fizzled out. They got back together last year for a few shows, but that was it. Renouned producer Andy Sneap was one of the founding members of this group.
Guns N Roses is another one. Still waiting for the 'real' follow up to "Appetite for Destruction".
Another Sepultura album with Max,Igor,Andreas, and Paulo would have been awesome, but of course they broke up after the "Roots" album in '96.
Megadeth with Marty Friedman would be kick ass too.
Dokken with George Lynch.
Queensryche with Chris DeGarmo on guitar.
Wintersbane only made 1 album "Heart of a Killer" w/ Ripper Owens, then he left them. That was a great lineup.
Deicide w/ Eric and Brian Hoffman, (the original guitarists.)
Cannibal Corpse w/ Chris Barnes, that would fuckin rule!!

There's so many more bands, I could really just go on and on.
What bands would you guys like to see make another one with certain band members or producers?