There's also the fact that there are multiple types of IO builds you can make.
Just off the top of my head:
1. Frankensloted with Commons and sets
2. Full Set IOs
3. Purples and Set IOs
4. Attuned and Purples.
Which do you balance around? Do you assume most folks are going with 1? Great, but then the content is still trivial for those with 2, and sure as hell is for groups 3 and 4. Do you balance around 2, still trivial for 3 and 4, and slightly harder for 1.
I don't think folks are realize how much a puzzle this could end up being.
EDIT: And I didn't even mention proc monster builds.