Let’s see…from the characters we already have at the 5* tier, I think just based on raw power potential the only true 5*s are:
Thanos
Silver Surfer
Gladiathor
Black Bolt
Phoenix
Banner
Dr Strange
Iron Man is tricky, because his super engineering skills and resources mean he can build a robot suit that will put him on par with at least Hulk, so I’d say Hulkbuster should maybe have been the 5*
Gambit has some serious powers, even if he is not a serious character, but I’d put him at best at 4* teir.
Spider-man, while wildly popular, is but mildly powered abd no match for any of those “real” 5*s. Black Suit Spider-Man, if it’s the symbiote, was more powerful, but still not on par with those others. You’d never send spider-man to deal with Thanos and expect him to come back
Green Goblin is somewhere between iron man ans spider-man - lots of resources, mildly powered, but still not a cosmic threat
Doctor Octopus is pretty joke-tier as well; sure he’s very smart, but he just doesn’t seem like a cosmic threat
Archangel is maybe over-rated? I mean sure he can fly, but his only gimmick is shooting razor blades from his wings, right?
Old Man Logan - the weakest fictional variation on wolverine maybe ought not to be the highest tiered one, though he does go toe to toe with Old Man Hulk in a pretty spectacular way in his comic, so maybe he gets a pass?
Captain America is an awesome character, but he is also not at 5* power levels. He would absolutely go fight Thanos, but he would do so expecting to lose, and he’d do it anyway since it was the right thing to do. His combat skills are decidedly top-tier, so he is going to undoubtedly punch above his weight so to speak, but at the cosmic level?
Black Panther is probably around this spot also; lot of resources, but not at the cosmic level
Daredevil has sub-spider-man level powers at best, and is decidedly street level. Not a 5* to me
If you are a street level character with no-to-weak powers (hawkeye, black widow, Star Lord) you don’t belong at 5*
Characters that maybe belong here but aren’t:
As mentioned, C4rol and Switch are probably 5* level powers, I guess Loki too. I’d put Nova in that list, and given her power siphoning capabilities, maybe Rogue could be as powerful as her opponent?
Characters that are tragically under-tiered:
Dr. Doom - serioiusly, he has to be the baddest earth based villain, right?
Magneto - Is he the most-powerful, non magic mutant?
Ghost Rider - he is a straight up for real demon, so he’s got some powers