Mount and Blade: Warband has multiple incredible total conversions. I’ve dumped a lot of time into Prophecy of Pendor and The Last Days, probably more than the base game.
For actually free games there are so many options that it really comes down to taste. Unciv is a fantastic reimplementation of Civ 5. Super Auto Pets is a fun casual auto battler. HoloCure is a really good Vampire Survivors-style game themed after Hololive vtubers. There are tons of MMOs and shooters that are F2P and good, but I know most of those from hearsay rather than experience.