I still think Starfield has the biggest modding potential out of all of their games. That's because there are so many systems in the game from typical RPG gameplay to space combat and crafting and base building. The fact that there is basically an unlimited amount of map space means you can load up a bunch of custom maps into your game. Custom ships, custom races and aliens... Am I the only one who sees this potential?
There's some potential but I can't really imagine where people would start. We could fill in stuff that seems to have been planned for the game but which Bethesda was too lazy to develop - make the Brig ship part actually do something by letting you send KO'd enemies there, for example, and make Unarmed actually work - but in terms of modding the planets I'm not sure what the point would be. The planets could be very slowly and painstakingly populated with Skyrim-tier dungeons which would be a big step up over the base game but still wouldn't surmount some of the core issues with Starfield.
Morrowind, Oblivion and Skyrim were easy to mod because they offered outlines of good games which could be filled in with mods, whereas Starfield is just a sort of jumble of shit that's impossible to sort through, let alone build on. Like, with Morrowind you can say "the problem is boring quests, let's create some better ones", with Skyrim you can say "the problem is dull combat, let's make ways to spice it up", but with Starfield the problem is on the conceptual level and the best you can do is work within those confines.