Enderal for a whole new world in the Skyrim engine. New map, new creatures, new spells, new items, pretty much nothing is unchanged and it's really well done.
Kingdom Come Deliverance for a more hardcore and true-life fantasy RPG
Oblivion if you haven't played it, just needs a texture mod for the most part.
Ultima VII - with the FOSS Exult engine, it's old but has AI schedules, and stuff that really reminded me of TES.
Baldur's Gate 1 - it's quite old and much more linear in many ways than TES, but the semi-open world was still great. The Pathfinder games are also quite similar for a modern alternative. Avernum is also a little bit similar for the open world aspect.
Thief 1 + 2 - first-person stealth games, this directly influenced Oblivion. The Dark Mod also has some good newer maps.
Deus Ex (especially the original 2) - first person immersive sims.
Kingdom Come Deliverance - the closest to TES overall.
Daggerfall (Daggerfall Unity), Morrowind (OpenMW), Oblivion - if you haven't played them already.
Arx Fatalis (Arx Libertatis) and Ultima Underworld - share some aspects with TES although are much more dungeon crawlers.
The Legend Of Zelda: Tears of the Kingdom - quite different but the large open world and focus on exploration is amazing.
However if you want something new, eurojank games tend to scratch same itch to me if you can tolerate unpolished game mechanics until you get used to it. Two Worlds 2 and Elex 1 is great games to start with.
It's basically better talent trees, not all that different.
For an actual different experience you need mods like Apocalypse (155 new spells) and most importantly the mod that instakills you if try to play a stealth archer.
I usually cycle through other Bethesda games, including but not limited to Fallout 3+NV (Tales of Two Wastelands) and Morrowind with OpenMW. I recently played Oblivion and Morrowind with vanilla plus mod list (this one for Oblivion and this one for OpenMW) and it was very pleasing.
If you're interested in medieval combat, Kingdom Come Deliverance has my favorite sword fighting system in a game.
For open world, Legend of Zelda Tears of the Kingdom did not betray all the positive reviews and I enjoyed it greatly.
Might not fit your bill but I would also recommend CDDA and Elona, both are excellent roguelikes with open world aspect.