The problem is balancing the effect/value of the base materia into the Tier of the produced, and balancing the value of each return per level, per HQ.
I.E.
If one gives you 200 MP and one gives you +5% MP per regen proc, what should the Refresh proc be? What about 400 MP and +5% ? Or 200 MP and +10%?
The Materia will have to be tiered off, and from there, you'd have to generate the Tier value for the Refresh.
Problem is, if 200 MP costs 100k, and +5% costs 50k, you're going to see Tier I Refresh cost 175~200k, easy... Then the problem of how close to Tier II value should be the Tier I +1/+2? Because of FFXIV's high numbers, it has to be % values for refresh, because "10/tic refresh" means drastically different things between classes -- and even between levels. Auto-Refresh in FFXI, even at 1mp/tic, had a constant sort of value due to how slow the game is. FFXIV is much faster than that.
And I'm not a fan of low level things costing that much, as well as I feel the balancing act could be complicated. That's really messy, but I think most of you will get what I'm talking about.