I’ll second the Barsoom suggestion. That recent flick was interesting, but it telescoped too much of the series into one movie and ignored ERB and made its own world.
I have always wanted an animated version of The Silmarillion. I have imagined it as a sort of anthology, where different directors and animators would tackle discreet “chunks” of the story. Some parts would be single-episode, and some multi-episode arcs.
For example - I’ve always imagined the Creation of Arda as a Fantasia-like animated episode with dramatic orchestral music and somewhat abstract visuals. Whereas, something like the story of Beren and Luthien would be a much more straight-ahead, conventional story of love and danger and adventure spread over a few episodes.
It’ll never happen, but it has been one of my projects for when I accumulate Elon Musk amounts of money.
Back in the 1930s animator Bob Clampett (famous Warner Brothers animator who went on to man the puppet show “Time for Beany” and the animated “Beany and Cecil”) proposed an animated “John Carter of Mars” that never took off. But we have some of his aimation samples on Youtube:
This was “naturalistic” animation in the style of the Fleischer studio’s “Superman” cartoons, but done before the Superman cartoons!