DM = Damien - It's been discussed it before, and there certainly seems to be some foreshadowing in the comic itself, but I think Damien would make a really good DM. That said, this is the shakiest of my guesses, because the whole campaign world is definitely Manny's baby, so it's entirely possible he'll still be at the reigns.
Korra = Shelby - Partly because Korra displays near full avatar power over elements from an extremely early age (something Aang noticeably doesn't do) and we know that Shelby makes crazy strong builds (citation - Saitama), and we see Korra going full pelt out of the gates in the series rather than taking a more cautious approach. It's
possible it'll be Mac instead, but I think Shelby's most likely for reasons I will outline with the next PC.
Asami = Lex - I think this is my most confident one. Lex is looking for ways to be competent that aren't based around shooting blasts of elements at people, and he's been doing knowledge: batman recently. Asami fits this nicely. Plus, there's Korrasami! Who doesn't want to ship Lexby?
Mako = Mac - Only tentative on this one. Mako was notoriously wooden early in Korra, and we know that Mac is roleplay deficient. Also, Makorra was a thing for a few seasons, and we know Shelby/Mac have some history together (I've read it as potentially romantic, but I may be misreading). Also, and something I just noticed, Mac/Mako sound similar, which would fit Mac's lacklustre RP habits.
Bolin = Rick - This is more or less a guess if I'm honest. If Damien isn't DM, Rick almost certainly will be I think. But Bolin seems goofy enough that Rick (in his lighter moments) could well play him.
Tenzin = Manny - Ousted from his DM position, Manny gets a bit stuck up about it all and is determined that the others all play properly
More seriously, I just see Tenzin as more likely to be Manny than Rick, and unlikely to be any of the other cast.
Jinora = Rube - I just get a feeling that Rube is going to want to play a more light-hearted character in Korra. Jinora seems like she'd be Rube's speed. Lots of RP potential, not necessarily combat heavy, just generally more chill than Azula.