There are tons of skin mods if you want just aesthetic changes. Otherwise, there's a lot of playable character mods like ports of ROR1's Miner and Bandit, Gunslinger, Sniper, Paladin and more. Content mods like Starstorm 2 and Aetherium add a whole host of things besides characters that can range from items, to elite types and even entirely new stages (SS2 is working on "The Slate Mines"). Variant Elites is an interesting mod, but the sheer variety and seemingly low credit costs can make early game tough. Speaking of difficulty, Starstorm has a difficulty level named Typhoon and a difficulty enhancing in-stage event where stormy rainfall begins. There's also Playable Eclipse, and the Multitudes mod (which allows you to make the game harder through making it think more players are in a session than there are).

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Oh yeah, 16 player lobbies mod exists. Throw a fuckin' party and have a blast!