Um um expanded world, acro/mach bike combo, more trick master, keep the contests how they were though! Faster surfing, more Dive areas, um um a few more casinoimean game corner games. More caves and hidden places! Above-ground secret bases need to stay/come back and you should be able to have a bed in there which acts like a PokeCenter. Better trees please but keep the general idea of the old ones. Lower encounter rate in the ash fields. Battle Frontier was great they could leave that as is really. Oh I can't decide if I like Contest Halls or Battle Tents to be scattered around the region though- I think Contest Halls would be best but by a close margin because I really like the big one in Lilycove that Emerald had. Ofc they could merge the highest-level Hall with the art gallery and put it up there too. Oh! I want more things to do with rope bridges and tree houses in Fortree; maybe have the entire place in a separate forest area? Like go in the forest and there is the city which is two or three levels in the trees and oh that'd be brill.
...whoops big paragraph :x but yeah there is SO MUCH COOL STUFF that could be done and I hope they expand without losing the charm that Hoenn had. I mean comparatively they weren't brilliant games, but they had some sort of appeal that attracted lots of people.