I agree that an animated series would be the best option for most of the cosmere. Most of the cosmere would end up with terrible CGI if it were a movie or live action show. But, I do think some of the cosmere novels could actually make a decent live action showing. Warbreaker would make a great movie and all they would really have to do for CGI is zombies, high contrast, and some flying scarves for awakening. Elantris could also make a good movie since makeup could be used for most of the movie with only a bit of CGI peppered in at the end. The only one that I could see as a good adaptation to a tv series is Wax and Wayne. The setting in Mistborn era 2 is relatively Earth-like, sure there are still mists at night but that wouldn't be hard to do with a fog machine and the ash mounts are gone so no ash to worry about. They could easily film it in the midwest. I think you are right about mistborn era 1 being impossible without a stupid amount of CGI that would end up cheesy.
Unless we have a huge advancement in CGI tech in the next few years I would never want to see Stormlight in live action. It has to be animated, and I think you are right about the avatar style for it. I saw a live action fan casting for stormlight and they cast tom hiddleston as Wit, which I think is as close to perfect as I can think of