What, you mean when he lost his powers in ROM? That didn’t have anything to do with lawsuits or anything, Nova was at that point a character without a series, and as tended to be the way things were handled in those days, another writer (in this case Bill Mantlo) brought his story to a close in another title, so he wasn’t just hanging out there as a loose end. And Nova didn’t become an Avenger in those days for the same reason that any number of other characters didn’t, from Omega the Unknown to Skull the Slayer to Deathlok to Spider-Woman: the AVENGERS creative team wasn’t especially interested in having them in the book (at least, not more so than the characters they already had.)