rooting is super super easy so I don't see why you wouldn't, especially seeing as you can both cover your tracks for warranty easily enough and OEMs often don't seem to mind if you've done that sort of thing.
However, in answer to your question... I'd expect almost 0 chance of a decent wide supply of higher capacity versions, why would they? It's pretty niche and when generally speaking people are looking for the phone "free" this just means the networks make less money per connection.
Either way if you use the Apple scale (as apple are pretty much the only company still that has consumers prepared to a) pay and b) pay more per capacity and finally c) have a customer base who expect the choice) you'd likely be paying £100 more for a 32gb version and £200 for a 64gb.
given the fact you can root and trim your OS and use micro SD cards I'd see those costs, any costs frankly as a non starter.
EDIT: not sure if it matters but on my 16gb version running wanamlite (mildly modified stock rom so you retain all the useful touchwiz stuff) and a reduced apps apk I currently have just over 9gb available to me, I think thats entirely acceptable for a 16gb phone and I could easily get more by removing some bloat manually as I have root access.
Worth considering!