Plenty of Skyrim mods add new followers, locations, questlines, or ways to look naked. Project Proteus is more about altering how you approach Skyrim, giving you an in-game editing tool to alter NPCs, items, spells, and even weather on the fly, as well as importing characters you've already created into your current game, and then switching between them as you please.
Some of what Project Proteus lets you do is already possible with Skyrim's console commands and existing mods, but this brings them together in a single pop-up menu. And while you could switch to another savegame to play your Khajiit archer for a while, Project Proteus lets you import them into an existing world state—meaning you can swap to a character with their own items, skills, and spells, but keep your current quest progression. NPCs who have died remain dead, items one character has left in storage can be retrieved by the other, and so on.
Plus, you can build your own adventuring party by importing whoever you want. Swapping between party members takes about 30 seconds so don't expect to smoothly bounce between them like it's Genshin Impact, but the idea of being able to hop to someone with a different skillset in different situations is intriguing. And being able to swap to a more appropriate character for certain quests brings plenty of roleplaying possibilities to mind.
You can download Project Proteus for Skyrim Special Edition and Oldrim, though it's not compatible with the VR edition. It's safe to install mid-game too, though make sure to check the description for a list of existing mods you'll need to have to get it working. It should be pretty compatible with other mods, even if you've Wabbajacked Skyrim to buggery with hundreds of the things.