>>1399558
There are a lot of good mods, most of which are either QoL or adding new (or old) content. For example, there's no Red Scepter in the base game, but there's a mod called Standalone Red Scepter that adds it and upgraded variants of the relevant skills for it. There's also Drop-In Multiplayer so people can join your runs mid-game if you've got a friend to play with; it even gives them a randomized pool of items scaling with the current run's power level. Aside from those, the mods that add back RoR1 characters tend to range from decent to excellent, particular the Enforcer. There's also Miner, Sniper, HAN-D, Pilot, Robomando, and an alternate version of CHEF from before the DLC added him for real. Noteworthy original characters are the Paladin, which was a cut concept from Hopoo's concept art, but people have added everything from Megaman X and Zero to Goku and Vegeta to actual fucking Gurren Lagaan. I recommend holding off on the super wacky stuff until you've gotten to enjoy the tone and setting of the game in its relatively unmolested state.
There are also some pretty large content pack mods like Starstorm 2 (and its spinoff Unofficial Videogame Mod 2), Aetherium, Sandswept, Lost in Transit, TinkerSatchet, BubbetsItems, etc. But they're a bit riskier to mess with because they're usually in a revolving state of broken or not broken depending on how recently Gearbox updated the game. Starstorm 2 is generally the most frequently updated.
If you want my personal favorite uncommon mod, it's Chen's Gradius Mod. It adds a couple items and drones that I think are cool.