Requiem Overhaul for Skyrim completely revamps how the game is played, you need to mostly pick an archetype, casting spells with armor on institutes a huge penalty in mana cost etc unless you pick talents for it that aren't available until late game, damage has been tuned way up for everyone so you have to ideally dodge most attacks and enemies drop loot that makes sense like their heart and some claws instead of finding 50 coins and a mace on a wolf.
Just installed it yesterday but the Multiverse mod for FTL basically turns it into a sequel and improves every aspect of the game. Go from 10 playable ships to around 100, have to actually consider choices instead of "I will blow everyone up for marginally more resources" and just an overall more Star Trek vibe with races having different factions etc.