Earendil, I always found his story interesting.
Beren and Luthien.
Yep, I agree there! Would love to see an adaptation for these two, plus the children of Hurin!
I think, if Amazon Prime wanted to produce a direct-to-video animated movie to promote their upcoming series, I'd like to see an adaptation of "The Tale of Aragorn and Arwen" from LotR Appendix A. However, the story of Elrond & Elros might be more appropriate for setting up the show.
I think I might lean towards the war in the north, a lot of the actors could technically still make cameos (if appropriate) because of their ages, and it is such a great unexplored area!
Obviously I would love to see other ages as well, but because of the timing right now, I would probably go more for that :)
The New Shadow?