when I brought up Poni Plains, I mentioned that as a worse case scenario. Poni Plains was a small and sparse field. if it had some kind of height variation or roaming pokemon, it'd be much better.
but my optimistic, but realistic expectation is, and forgive me for sounding like a broken record here, something like Dragon Quest or Monster Hunter Stories*
MHS had like 5 towns but the fields were pretty damn open with some dungeons. Dragon Quest 8 had 16 towns and the world design was linear but with a good deal of exploration
Large Map using the ingame assets, so you can see the main path and how much you can explore
https://www.woodus.com/den/gallery/...est_8_Overworld_Map_25_Percent_Resolution.png
these are well within the ability of Game Freak, I think. and we all know I don't have too high of an opinion of them.
*christ, I bring up these two games a lot, but I do it for a reason. a lot of the problems I have with Pokemon have been handily solved with these two games.