Predicting is tough. NRS has a history of swinging fast and loose with the nerf/buff bat. What I think would be reasonable and will overlap with some of what they do is as follows:
Cassie: A few buffs to her drone move(s) to make them worth thinking about using.
D'Vorah: Frame buffs, about half to 2/3rds back to how she was at release. If she kept everything she had then, she'd be OP.
Erron: Make Scud Shot cancel either have slower recovery so all he can do is block after, or make it cost defensive meter. Make F4's lying hitbox vulnerable to most lows. Make moves in second variation worth using.
Frost: Give her basic buttons decent hitboxes and startup times in line with the rest of the cast.
Geras: Forward throw bounces opponent too far away to follow up. Swap which variation has which sand trap, or else make IW's sand trap KB activate on the third counter hit. Possibly add a couple frames to sand trap's startup. Make f212 -4 on block. Slight damage reduction on 111 KB.
Jaqui: Slightly increased gap on her stagger pressure or else make it cost offensive bar. Make second variation competitive.
Jade: Slight damage increase and/or a couple decent combo paths off her faster pokes. She should be netting mid-200's for combos. Make sure second variation has better combo options than first.
Johnny: For god's sake fix his hitboxes! And KB requirements! Make that second variation worth something!
Kano: Hitbox fixes. Make second variation worth using. Give him a serviceable mid.
Kollector: Hitboxes and frames of normals need to be improved. Bag bomb tools should actually be threatening (decent startup/recovery, reliable popup, etc.).
Kotal: Give the man some mids, fix his hitboxes, and a little more speed. And for god's sake give his variations some actual synergy (totems together, etc.).
Kung Lao: Mix some mids in with his highs, give him an overhead. Not an amazing one, but something that's mildly threatening a la Sub-Zero's. Give him a block string or two that aren't super punishable.
Liu Kang: Nerf the frames on his ONE string right now, buff the frames of a few others to compensate. Give us a reason to use his second variation.
Noob: Second variation sucks. Please fix.
Raiden: Improve his pressure in first variation, and his zoning in second variation. Give him a safe way to block-confirm stormcell (e.g., make all hits mid but less pushback, etc.), and better frames on teleports that cost meter. Give second variation a midscreen combo path from the move that charges up on block. Several of his strings (esp. b12 and f3) need more reasonable frames on startup and recovery.
Scorpion: Teleport hits high. Must be MB'ed on startup, not hit or block.
Shao Kahn: Fix hitboxes, make his hammer swings decent, give him at least one safe, - on block cancel.
Sonya: Damage nerf off of mixup strings. Ring cancel costs defensive bar.
Sub-Zero: Delete first variation's mixup or else make it actually good. I'd prefer to delete it and give him more interesting, less busted tools. Doing so opens up to buff him in ways that won't be broken if he doesn't have a stupid 50/50. Give him a decent, combo-starting mid. Make second variation's icicle move a bit safer. Make second variation's other projectile good for something (probably faster startup). Give him a variation that actually has his fun tools in it.
Shang Tsung: Release him already!!!