It all depends on if anyone can create an importer/exporter for the Model Data
I'm sure a complete texture overhaul and character swapping will be possible since .tac files can be rebuilt (waiting for release)
I'm most interested to know what kind of Model data ShenmuePC uses I would guess its a simplified OBJ type, If they reused the original MT5s with internal emulation I'd be impressed and utterly annoyed
It seems that everything that was modifiable on the DC will work for PC
On Modding Ryo's body, his body mesh is still comprised of multiple overlapping body parts so swapping him for a fully skinned character wont be possible (but swapping him with another charater will be possible)
Hopefully the Model Data will be also modifiable and in that case I would remodel everything.
Also being able to edit Model Collision would mean ingame Landscape could be extended
Best case scenario would be if level data could be edited (interactive objects etc)
Either way a true HD Shenmue remake is on the horizon