So long as you own both Skyrim (or Skyrim Special Edition), Oblivion, and all their respective DLCs on PC, Skyblivion will be completely free to any interested parties, despite the breathtaking amount of time and effort its taken for the development team to bring Cyrodiil back to life. It’s called Skyblivion (Oblivyrim just isn’t quite as catchy, and sounds like something a doctor might prescribe you for the sniffles), and it’s currently under construction by a large network of modders led by the TESRenewal group, a community also responsible for the Morroblivion and Skywind remaster projects.
Not a crossover per say (though the idea of dragons soaring across Imperial City does sound appealing), but something potentially even better a fully remastered Cyrodiil made in Skyrim’s own game engine.