Does anyone remember Mini Fighter?
The PvE was quite similar to the usual side-scroller MMORPGs back in the (PvE gameplay) but what made it really unique was the PvP that played like a less complex fighting game (PvP gameplay) and a character card system where you could switch between different ones as long as you had their cards, it involved leveling and upgrading them which increased their invidual stats. Even though it was somewhat p2w and RNG, it made the game's economy really fun and interesting and it was really satisfying beating the paying players with your low level characters once in a while as a kid.
It was such a cool game, wish there still were some EU/NA servers but it seems like the only ones that are still functional are some private Brazilian ones. I wonder if there are any similar games nowadays.