I'd like a metroidvania, or maybe a roguelike
Going through areas of Equestria, collecting items and spells that would help you explore and fight. The Everfree Forest, Diamond Dog Mines and the Castle of the Sisters could be cool settings. There's even some cool unexplored settings, like Mount Everhoof and the Flame Swamps with the Chimera!
Honestly though, I feel that if there were to be a pony game, it'd be really hard to make it about the mane cast; they're too powerful, and the show's already shown them in 101 situations. Best protagonists for the game would be either original characters for the game (Them's Fighting Herds style), or perhaps secondary characters, like the CMC?
Overall, I just really like the idea of exploring locations and finding items from across Equestria.
I've just been watching a video of Noita (a physics-based roguelike), and it seems so cool, and that's intrigued me a lot.