I had some thoughts an ideas on this...
Destroying a boulder with the hammer should turn the tile into road rather than desert.
And speaking of roads, how about if you run into an enemy encounter on a clear road the enemy should disappear instead of triggering an encounter? Examples of what I mean are like when you walk into an enemy while you're walking on the ocean with the boots.
To make Rock tiles turn into another type of tile, you need to do two things:
Each tile that can change to one another is defined in these two tables:
1DF5E: 0E 0F 04 06 Table for OW tiles that transform into another type
1DF62: 04 04 02 00 Transform into this
To make Rock tiles turn into Path tiles, change 04 at 1DF62 to 09. This modifies the byte value to change to in the Map Data in VRAM. It does not apply an instant change on screen. To do so, you need to modify the PPU Macro at 4395 (West Hyrule) and 8395 (East Hyrule) :
4395: 0A 00 00 82 6C 6C 00 00 82 6C 6C FF
Change values at offsets 04, 05, 09, and 0A (all 6C in the original) to whatever Tile Codes you want to instantly appear when a Rock is destroyed. If you want a path Tile to be drawn, replace all 6C values with FE.
Personally, I prefer the Desert Tile because it acts as a visual cue that the Rock has been destroyed, and it looks like there's only pebbles remaining. The Desert tile temporarily inherits the yellow Palette Code of the Rock tile, so it looks like a rocky Path tile. The downside is that after you get far away enough and come back, or switch to a Side View area and come back, the desert tile returns to its normal Palette Code, and turns pink.