auto input switch (qol when playing on an ergonomic Xbox controller on the PC)
cutting room floor (increases some content based on unused assets in the base game and the mod author's arbitrary judgement)
convenient horses (makes horses nicer and this plays nice with other relationship mods)
nethers follower framework (allows 20 followers at once and can configure follower outfits and can have followers loot for you)
unofficial Skyrim se patch (mod author made changes that make Skyrim a little less buggy based on how the quests seem to make you think they work)
unofficial ae Skyrim patch (arbitrary tweaks by the mod author to make ae less buggy)
SkyUI (user interface that is more efficient than the base game and adds a menu to the start menu called "mod configuration manager aka mcm" and it allows you to configure any compatible and configurable mod.)

You can build off of that in a lot of ways. There is also a nemesis engine that is better than FNIS (allows true directional movement, some combat mods and new npc idles)

You can also get some weather mods that change the feel of the game.

You can also get some lighting and shadow shading mods.

You can also get some content expansions (serana dialogue overhaul, wyrmstooth, falskaar)

There are also some mesh improvement mods (high poly, smim, etc.) To make the walls and floor less jagged.

If you want to change context, you could get one of the "alternative start mods"