It was only a matter of time. There are two decades' worth of mods for the original Diablo 2, and plenty of reasons to start tinkering with Diablo 2: Resurrected beyond its graphical overhaul and shared inventories. The mods that have started appearing on NexusMods and ModDB are doubtless only the beginning.
There are difficulty tweaks like Enemy Multiplier, which lets you increase the number of monsters by anywhere from double to 10 times as many, and Lower Resist, which replaces elemental immunities on Nightmare and Hell difficulties with resistances. D2R Drop Mod and Guaranteed Drop Mod both let you mess with drop rates, the former increasing the likelihood of getting good loot from tough enemies, while the latter just makes every single enemy drop something. Splash Damage to Melee makes nearby enemies take half the damage you're dealing. Hide Helmet, well, that one hides your helmet.
Better SP is full of quality-of-life improvements like increased inventory and stash size, faster walking and running (as well as reduced stamina drain), higher stack limits for arrows, bolts, keys, and tomes, higher mana regeneration, Horadric Cube recipes for respec tokens and unsocketing items without losing gems, higher max skill levels, changes to the secret cow level, and more. Sounds great—now if someone could just mod in a decent modern hotbar for using your skills.
Note that most of these mods warn they're intended for offline singleplayer games, and that if you get banned for using them that's tough cookies.