As for the mountains, the 3rd Mountain Set (Oracle of Seasons) would suit best for this setting, as it matches the trees perfectly and makes it very GB-like. As for the water, you can use the already existing water boundary tiles, but re-colour them to the grass. Or you could do this:

I'm aware that they don't use the same tiles and the colours are different, but the style I'm talking about is basically what's above.
Also, one more thing about those water tiles. It would be best if you change them to these:

These look more like proper water tiles. The one you are using right now suit more for shallow water (even though they are set to water combos, it honestly shouldn't be)








