Overhaul Candidates
This is a frequent forums topic, and deservedly so. When I look at the fleet of 6*’s, though, I’m convinced the number of champs who need a complete overhaul isn’t as large as one might think. 
When I look at my roster, there are plenty of underperforming champs. But many of them could be improved with basic numbers adjustments or via the addition of a simple buff/debuff/limited immunity type of change. As an example, take Beast: not anywhere near the top of the mutant class, but also not totally irredeemable. Numbers adjustments, maybe a shrug mechanic or the ability to apply some more debuffs (a la MrF) would make him a decent utility champ. But I don’t think that rises to the level of an overhaul. 
I also don’t believe the purpose of an overhaul should be to make a champ one of the best overall in the game. Rather, the goal should be to bring a champ into the current state of the game and make them a viable choice for at least some of the current content. 
So here’s my list of top overhaul candidates by class. I’m sure opinions will differ—but these are the most urgent in my opinion and in most cases the most long suffering. 
Skill
Karnak
OGBP
Cosmic
Groot
SIM
Drax
Mystic
Iron Fist
Juggs
Science
Joe Fixit
Ant-Man
OG Spidey
(Miles is omitted because he is a certain overhaul case)
Tech
Iron Patriot
Howard the Duck
OGIM
Mutant
Unpopular opinion I’m sure, but I’m not convinced any mutant needs a total overhaul. Both Cykes and DPX would be top of list for me if anyone, but I tend to think values updates and minor tweaks could make those champs useful again. I wouldn’t object to an overhaul—but I think the mutant class generally is a strong one top to bottom. 
Dr. Zola