The very first big nerf was Loki. This was back when it took 60 covers or some nonsense to cover a character, before my time, but as far as I ever learned, he had an ability called Transmogrify instead of Illusions, which basically did was Thor’s Thunder Strike does now, except with no damage component and 16 purple tiles instead of 9 yellow. Not sure on the AP cost, but I suspect it was 5 purple, like Illusions is. So basically a self-fueling winfinite, and completely brainless.
Ragnarok has been buffed twice, including getting a complete redesign complete with a third ability, since his big nerf, which happened extremely early in the game’s life cycle, compared to the others people have mentioned. The original Rags was also 1-character winfinite: 2 red ap to turn 5 tiles of your choice to green, and a 6ap green nuke with board shake. Basically completely unrecognizable from how he is now. He could definitely beat 5*s once in a while if he went back to his old configuration, since there would be times when he just never needs to give the opponent a turn. It would take a long time though.
2* Wolverine was the meta for a good while after that, with his 3 AP green and a heal that was always on. Obviously that wouldn’t be a problem now. Honestly it wasn’t really a problem then, except that matches tended to take long enough that his heal was oppressive.
They then got rid of environment tiles, and with it, the 1* Storm winfinite. She used to have a red ability that interacted with the background, and if you were on a desert background then she could basically chain it forever. They could not bring her back at this point just because the game doesn’t support the mechanic anymore.
Spider-Man was the meta for like 4 months after that, (2AP 3-turn stun, woo) before he got nerfed. This was about a year after the game had come out. Viable like that now? Probably not. 2AP for a long stun is pretty insane.
After that, they nerfed true healing, which was primarily a nerf to OBW, who was being used to prologue heal entire teams. They could bring back true healing, but it wouldn’t make her more viable. It would just make the game more grindy.
Then they nerfed Magneto, the rearguard of the cheap self-feeding battery powers: 9 blue to make 2 blue tiles, and a red for 6AP to board shake. I still think this one was a mistake. At those costs, it was never sustainable for more than a few cycles, so it was never a winfinite like Rags or Loki were. It was just a fun, slightly-slower-but-more-sustainable alternative playstyle that happened to work super well with Patch. Bringing it back wouldn’t really mean anything in this era, since he wouldn’t even come close to the sustainability and speed of Dr. Strange. Although people might like him back just for nostalgia. He was super fun to play.
The last of the “old guard” nerfs were to Hood and Sentry, together. They made PvP shield-hopping trivial since every match could be ended in 3 moves, if you brought proper boosts. You could bring 6AP worth of of every color at the time, and all you needed was 1 green to fire World Rupture, and 2 yellow for Sacrifice, and 3 black for Intimidate, and thanks to Dormmamu’s Aid, you could pick it all up by turn 3, or even turn 2 on a good board. It was enough to win the turn after, every time. Bringing both of them back now would just not be a big deal, because boosts don’t work like they used to, and their combo doesn’t apply in a world of triple-size health pools.
