I'll start off with one.
The Vision.
This bot has been sporting nearly the same appearance and powers as he did when they created him. Being a synthetic man, you would think that they could come up with better tech for him.
If it were up to me, I would let him keep all of the abilities that he has, but add on that he was given a new body made of nanites, and had the ability to launch swarms of deadly bug like mech insects. A strength upgrade would do him good as well.
I would give him gravemetric shielding to shield against e EMP assaults that would shut him down, and further help to increase his density manipulation.
I think that they should bump Captain America's strength level up to class 10 or 20, and just write it in as a mutation of the SS Serum, which also makes him age much slower.
Spiderman could go for a new costume in the style of Night Thrashers.
Sometimes they toy around with making him awesome like they slowly start to make him what he should be then boom one thing happens and he's right back to where he started out.
Few have been through as many costumes as Pete. Hell, in the last 30 issues alone he's worn a Tron-like tech suit, the classic suit, the Future Foundation suit, and I think he may have slipped into the black costume (non-symbiote) not long before that. There's also Kaine's new look and Flash wearing the symbiote like a military uniform. I think this isn't needed.