This might reflect moreso my own wants but here goes. I've tried to space out the whole fellowship across a wave, plus some characters and locations that were left out in the OG sets.
Fellowship of the Ring: Boromir's Stand.
Probably the least likely one given how dark the subject is, but I was struggling to find any other scene that would provide the characters I was missing.
Builds are the ruins where Aragorn tells Frodo to run, along with some additional trees and foliage.
Figures are Aragorn, Boromir, Merry, Pippin, Lurtz, and two additional Uruk-hai.
Two Towers: Nazgul Attack.
I'll be honest, I mainly just want a Fellbeast.
Builds are a small portion of Osgiliath and a Fellbeast.
Figures are Frodo, Sam, Gollum, Faramir, and a Ringwraith.
Return of the King: Mumakil Charge.
Another creature build that I want to see (there may be a slight pattern to this).
Build is a Mumakil.
Figures are Legolas, Gimli, and three Haradrim.
The Hobbit: Smaug.
I think Lord of the Rings adopting sets similar to the Hungarian Horntail or the Tallneck would be a great way for some smaller but still displayable sets.
Build is Smaug with a base of piled-up gold and jewels.
Figure is Bilbo in his blue coat.
Diorama: The Bridge of Khazad-Dum.
Plenty of people have already expressed a want for this as a set in this thread.
Builds are the Balrog and a portion of the bridge built on a black base with a "You shall not pass!" quote tile.
Figure is Gandalf.
I'm not good with imagining hypothetical prices and piece counts, so I'll leave that to anyone else if they so wish