I was going to suggest having the forest near the ghost gym, as I've managed to think up some plot points for my character, but I'm not sure if I should suggest them here or over PM. I don't know how many plot details you actually want to put in here.
We need:
Obligatory large forest.
Obligatory underground tunnel/mountain tunnel.
Obligatory island town with a gym. (I think Shinnoh is the only one who didn't really do this, but even then, we got the Battle Frontier on an island. )
Not sure what else we need.
Sinnoh also had the island with a battle area/survival area/resort area etc. At least, it
seemed like an island.
Anyway, so I'm guessing there's going to be some water routes as well, then?