I haven't really been paying attention to modern builds, and everything is still being figured out anyway, but here is the general idea afaik.
TvP: Bio/liberator. Play defensively/greedily initially, then harass with drops and finish him with a doom drop + liberators. From what I hear, that should end the game.
TvT: Tankivacs and bio.
TvZ: Definitely bio over mech. Might want to kill him quickly before ultras come out. I think liberators are popular. Need either a lot of liberators or ghosts to deal with ultras if they do pop (marauders don't do anything to them)
Also, as a bit of encouragement, I know a low GM player who took a 9 month hiatus and after about a month is already back to top masters.