Whatever people want to make. The Switch is completely hacked at this point, by default even (no split like we might have seen on the 3ds or 360), and there are similarly advanced tools for debugging (not as nice as some emulators but still good).
Pokemon has traditionally seen more tools made that allow people without hacking skills to edit the games, this does seem to lead to a lot of people doing some kind of "I changed the starter" and when the ability to set what pokemon appear in a given area appears then you will get the all pokemon in the game type hacks.
Most likely the earlier hacks will be stats based, map changing based, maybe some text tweaks and fiddling within the existing parameters, possibly a few graphical hacks as people twist whatever ripping options are available get twisted back around. Sound hacks also likely in this but as sounds on modern consoles are a glorified zip file with known formats that is not quite as impressive as it used to be. You might get some people working within said parameters (I did see a very nice boss rush type deal there before in a hack, and you could hardcode what is effectively a nuzlocke challenge this way too). From there you will tend to then see people hardcode cheats, and follow what instructions are doing what to the addresses those cheats have so you get into early assembly hacks. Off to the races from there really, though when this might be could vary dramatically as one person with some skills (and while I am not necessarily the most charitable above regarding "I changed the starter" I have seen some very skilled people playing in pokemon hacking circles in the past, and going by the language and methods used largely self taught and self discovered) can advance things very far, very fast. Somewhere in all that what the pokemon hacking world knows as scripting (that is to say changing say walking patterns of AI, or what triggers when you stand on a certain tile) and everything else knows as events and behaviours will probably also get looked at.
I could detail an approach you could consider if you wanted to make it all happen faster (I have basically no interest in the Switch and even less in modern pokemon), and some of the modern programming languages make life far nicer than those that might have had to consider visual basic back in the day (if you have had to install odd things to play with tools to fiddle with GB/GBC/GBA pokemon games then probably was that). I am not sure what good it will do for you though.