Here's my pick for a new Western version (includes some games from the current Asian/Japanese versions):
Sonic 3&K

Rocket Knight Adventures (surprised this wasn't on the first Mini)
Streets of Rage 1/3 (I would personally go for 3)
Golden Axe 2/3
Quackshot
Aladdin
Teenage Mutant Ninja Turtles Hyperstone Heist
MUSHA
Hellfire (rock hard, but underrated shooter)
Thunderforce 2 (number 4 is the best, but I don't think Sega want any overlap with Sega Ages)
Outrun/Super Hang-On (the original was sorely lacking in driving games)
Desert Strike/Jungle Strike
Alien Soldier
Mickey Mania (although I'd prefer they save this for a Mega CD mini)
Sunset Riders (Konami already on board)
International Superstar Soccer Deluxe/FIFA/Sensible Soccer (needs at least one football/soccer game)
NHL (I believe there was a version without real teams/players)
Madden (see above)
Mutant League Football/Mutant League Hockey
Phantasy Star 2/3
Sonic 3D (they would definitely be more than one Sonic game, and they might be able to include the "deluxe" version patched by the original creator himself)
Ristar
Revenge of Shinobi/Shadow Dancer
Vectorman 2
Mortal Kombat 1/2/3
ESWAT: City Under Seige
Crusader of Centy
Alien Soldier
ToeJam & Earl 2 Panic on Funkatron
Shining Force 2
Earthworm Jim 2
Super Monaco GP
Battletoads (maybe Microsoft will play nice, since Sega is helping their new PC gamepass project)
Afterburner 2 (good arcade port)
Alien 3 (don't Sega still own the rights to Alien?)
Super League (unlicensed EU/JAP version of Tommy Lasorda Baseball)
Ecco: Tides of Time
Gauntlet 4 (M2's first ever game - surprised not to see this on the original Mini)
Micro Machines (great multi-player fun)
Mystic Defender (underrated early action-platformer, but licensing?)
Snow Brothers (already on other versions)
Ranger X (really need to 6 button controller for this one)
These are some I had to cut out from my original list (you can see a slight Amiga bias here, haha):
Theme Park
Lemmings (timeless puzzle game, earns extra nostalgia points)
Speedball 2 (another Amiga game - futuristic sports)
Cannon Fodder (really fun Amiga port, RTS/action)
Megalomania (another Amiga RTS)
Syndicate (really cool strategy game)
Road Rash 3 (didn't make the cut because it was a bit too similar to Road Rash 2)
Forgotten Worlds (another decent Capcom arcade conversion)
Zombies Ate My Neighbours (never played this, but it looks interesting)