A friend is playing necromancer and has pointed out a minor annoyance with the UI that I think is pretty relevant: It can be annoying to keep track of whether or not you have the maximum number of summons possible when you’re playing a build that relies on minion quantity rather than quality.
Skeletons come to mind as the most problematic minion type for this, having to glance up to the corner of the screen and figure out how many of each type you have (and if they are from the skill Summon Skeletons or Summon Skeleton mages)
My initial suggestion would be to either add a counter or border to the skill icon on the hotbar itself, or perhaps add it as a buff so that the information is still grouped near other pieces of the UI you care to check regularly. This would purely be to indicate if you had the maximum number of summons you can for a particular skill in some way.