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If you limited it to just the first 4 regions, you could probably get away with the regular lvl 100 cap. Even limiting the game to the first 4 regions, you are going to have to bend over backwards to make a good game out of it.
1.) You have to delineate the regions from their original order. Just playing the regions, one after another, is the same as just playing the games individually.
2.) Instead, you have to really treat the 4 regions like one big region. This means making extensive use of surf, SS Anne, Trains, and Airports to shuffle the player all around the MegaRegion.
3. A lot of each region is completely redundant. Each region has an early cave, forest, empty starter towns, breeding places, etc. We have also explored all these places before, so you would be retreading a lot of the same ground. You'd have to give each region a makeover to re-purpose a lot of these towns and dungeons. Expand Mt. Moon and make it a late game hard cave, for example. Add Dive to other regions, etc. Something to make these places worth revisiting.
4. Since you are changing the core regions and games so much, you need a new plot, obviously.
All that is a lot of work. No one is going to do all that shit. But it wouldn't be a bad game, in principle.