It might be kind of unnecessary for a game, but I'd be interested in seeing a game detailing the events that built up to Spirit Tracks and the settling of New Hyrule. Malladus as an actual threat would do some good to separate him from the other non-Ganon villains of the series, since he's supposed to be the big evil of New Hyrule and if they do plan to ever have another adult timeline game, it would be a better foundation for him to give him more of an origin story. Spirit Tracks prequel might also work as a decent transitional point for the series' relationship with the open world stuff of Breath of the Wild, and ease back into a more structured game while still having the openness of a land unexplored to satisfy fans of both.
Alternatively, anything pertaining to Lorule and/or the destruction of its Triforce would be amazing.