17's aren't bad. As far as tire sizes 265/70-17 is basically identical to 265/75-16, 285/70-17 is basically identical to 285/75-16, and 255/80-17 is basically identical to 255/85-16. So you really aren't limited by the 17's at all. When I buy wheels I'll plus size from my 16's to 17's on my Pro4X.
If you go bigger on tires your best bet is to stick with the stock wheels with 30mm offset or aftermarket that are really close to stock. This would mean avoiding wheels like Level 8 MK6 and Pro Comp 7089's, both of these wheels stick out of the fender quite a bit further than stock (30mm and 42mm respectively).
I'm not positive what will fit stock without any work to the inner wheel well or lower fender (metal). It seems like the less offset a wheel has the more work people need to do to fit larger tires. I would check out clubfrontier.net for more info on what will fit at stock height.