About expertise dice. What these do is they introduce a mild bell curve (atcually a flattened pyramid curve with one small expertise die) and I like bell/pyramid curves. But adding them to a d20 system seems to be outside the d20 paradigm. To me, expertise dice feel half-assed. Are you sure what you want is not to go to a system that is entirely about random distribution curves? Like 2d10 instead of d20? I can see how the d20 is a sacred cow, but I am not sure expertise dice is the cure. Then again, I have not played with expertise dice. What I have played with is the Wild Die of Savage Worlds, and that works spectacularly well.
I also loathe the Earthdawn phenomenon where you could not roll until you know of all the modifiers because the modifiers changed what kind of dice you should roll. That does seem nopt to be the case here, but I thought it worth mentioning as an example to avoid.
Again, these are simple viewpoints, not demands.
Forgive me if I missed something, but wouldn't these expertise dice be just like the variant proficiency die option in the DMG? If yes, then this expertise die is firmly sticking within the confines of 5E.