Theming it to Tom Sawyer/Huck Finn or Lewis/Clark makes more sense tbh. Another generic 'wild mountain' wouldn't be the best addition... especially when Frontierland is the one area of the park that doesn't need something with a height requirement.
It's days like these where Tomorrowland or Adventureland should be getting an overhaul/expansion. Both of those are so clear-cut/obvious.
Adventureland
- Overhaul Jungle Cruise (rework ride path)/Tiki Room
- Rework infrastructure to allow new guest path
- Expansion behind POTC/JC
Tomorrowland
- PeopleMover/CoP/Space/Buzz overhauls
- Stitch replacement
- Laugh Floor replacement (expands into MSUSA parking lot)
- New attraction between Space/CoP, on top of CoP
- Speedway/Cosmic Rays space used for Fantasyland, space behind Speedway/Space used for Tomorrowland
Frontierland really can't add much, unfortunately. Either, a raft ride, Geyser Mountain, or WRE. Possibly a stage show. There's not a ton of directions they can go, unless they build a new mini-land in that expansion pad.