A random thought, just winging it out there:
What if Bonus Skill Level affixes had two modes? Keep the one we all know and love, that provides +1 to +4 to our skills, with some bonus fitting for them. Then, we have a second variety that provides “bonus max level” and eliminate the secondary effects for these affixes, say with a range from +2 to +10 [+2, +3, +4, +5, +6, +8, +10] - allowing players who enjoy the xp grind to push some of their stretch-goals into gear affixes.
The main point would be that these affix-types only increase the max level - not the current skill-level, so players need to continue levelling their skills after adding them. And for an example of an item that would greatly benefit from this: Call of the Tundra. With it’s wide range of “bonus level” affix for Warcry, and the inherent -1 from forcing you to allocate a node that no longer has any effects to get to the nodes behind it, it would benefit greatly from a much wider range of available levels.