Well, I suppose you need to think more about the plot of your game then.
https://bulbapedia.bulbagarden.net/wiki/Pokémon_Professor
Just for reference. It also goes into spinoffs and Anime which allows for a broader field of studies.
Gens 1 and 5 didn't really introduce anything special, so they study more general Pokemon Research compared to the other professors.
I guess Sonia too, but she studies legends and history, much like some other characters in the series, such as Cynthia.
But there are so many Pokemon topics to study in the series even when you can't turn to the standard of "New mechanic".
Perhaps a study of regional forms and how they develop if you have them.
Maybe something critical to the plot that the player needs an expert on?
Or if they don't have to be a Pokemon Professor specifically, they can study another field of science. I toyed with a meteorologist professor idea once.
I don't have any specific advice though since I don't know much about your game.