The concept is really great. Kanto and johto harder and with bigger map to encourage exploration and actually paying attention to type charts (like really ... on official game you just stomp everything with your starter and that's it).
Tho, I did find the lvl63 raichu at vermillion city too much of a difficulty spike. Pewter: Around level 20, Cerulean: Around level 25/30, Way to Vermillion: level 35 to 45, SSAnne: around level 40. Then Vermillion gym trainer being Around lvl 45 but Lt. Surge having a lvl 63 raichu. Sure, if you have something efficient against a raichu you could make it. The issue is if you actually don't Raichu is too fast and hit too hard. If you don't have a pokemon who resist thunder type you are basically doomed to get one shot. That wouldn't be an issue if the wild pokemon around weren't lvl 25 at max. I believe raising the wild pokemon's level to 30 at the map east of Vermillion would make it not being an issue so you could raise a new one faster.
Speaking of wild pokemon's level That is a point where a believe an improvement could be made. From cerulean city to Saffron city, wild pokemon's level don't increase. From badge 2 to badge 6. Then on the way to the seventh badge they suddenly become level 55 to 65 at max. Then they will keep being that level all the way to Sevii island for 12 badge and suddenly raise to level 100. I do believe it would be great to slowly increase the wild pokemon's level instead of having sudden raise.
You can feel that the further you get into the game the less the maps are polished but what did you expect ? This game is still in development so not an issue at all.
Another issue right now is that there isn't much story mission. Player could feel lost at many point but I do believe this will improve in the future just like the previous point.
Through the adventure, I've met lots of Lopunny spam Bounce and that is annoying as hell.
In Ilex forest, the 2 rival team up against you. While it's a nice battle I do think it doesn't make sense in a story pov. Rival2 just tried stealing rival1's pokemon as the later told you.
A thing I really appreciated was the ice badge, the 14th. Having to use the guy at the entrance to get to the gym master was fun. I also really appreciated the raid type battle 18v6.
Would be great to have ways to reset EV. likely at the EV trainer place.