I voted +3, but with a caveat. I want general bonuses (to-hit and damage, anytime; AC, any kind of damage) to max out at +1. +2 and +3 I'd limit their sphere of effect. +1, +3 vs dragons sword. +1, +3 vs cold damage armor. At least in the core rules. Later, you can add a high-level module that let's you recreate 4e Epic Tier awesome-crazy sauce, with higher defenses and thus higher bonuses for magic items. Although even then I think +5 is plenty high. In any case, keep the dice relevant, and not just tacked-on numbers to an astronomical to-hit and damage bonuses.