Well, I've done that before with that valley that was where Graal PD was readded (my only disappointment with that is that it ruined the giant fish-shape of the east mountain). You can chop down some trees west of Graal PD there and divide the east and west mountains again into a new valley. Extending the mountain-top areas north a bit would allow for a much more immersive effect. There is the easier way just to throw in a rail cave exit out of the north face of the mountain, but I would consider a valley to be the better option.

Also, if you'd be extending the cliffs northward, you could kill two birds with one stone and plan out some space to eventually readd Sylphia (Old Snow Town).

The current Snow Town mountain was initially built by someone else, I forget who exactly. I freaked out when I saw it because it was like the biggest tile error I ever saw. Looked like a giant LttP rupee on the map screen. I ended up pulling an all-nighter retiling the entire thing. So I'd estimate it's probably something like 16-20 hours of work to build a new valley and have space for Sylphia **if you're versed with cliff tiling**, and readd the Dark Forest and Ninji Castle Island. That's excluding scripting and graphics, which would be their own issue.