Evolution. I know it's already been covered by both Elm and Rowan, but… well, neither of them seemed to do much. Elm takes an interest in the Togepi egg, and Rowan once notes that Legendary Pokemon tend to not evolve; something that's no longer strictly true.
I'd like to find a way to synthesise the trade evolution process, without trading. Basically to get Pokémon evolution to the way it is in Legends Arceus; where trade-items just function as standard evolution items.
It'd be generally cool to do a deep-dive on why certain items evolve certain Pokemon. It'd also allow for cool opportunities to experiment, like… what if I eventually conditioned a Growlithe to evolve with an Ice Stone instead of a Fire Stone? That kind of thing.
I'd have a bunch of Eeveelutions running around the lab, and if I ever got to be a secret-boss-battle, I'd have Pokemon covering as many evolution methods as possible. Level-Up, Friendship, Stone, Trading, Flipping-upside-down… there's already more methods than one can have on a single team. So it's a cool, in-depth field of study, we just haven't seen them explore it much in the games.