Additional Mercenary Slot(s)

Since the mercenaries are account wide, and not character bound, having just a few more slots would solve so many limitations. As of right now, you can't really keep any mercenaries for lower levels if you have a few different builds going.

I'd say the cap should be at least 6. This way you can keep three for leveling purposes, and three for end-game purposes (one for each different build up to 3, or multiple for the same build).