I think the idea in the OP goes right past giving us Triplecast and tries to make us into full-on Black Mages.

I mean, overcapping your Mana lets you cast three spells of at least 600 potency each, in a row? And makes it easier to do again? You would never want to melee combo again.

Though the idea of a Triplecast has reminded me of a neat idea I once heard from Zynest: Just give us our own version of Light Speed, which reduces all cast times by 3 sec. Verthunder and Veraero become our shortcasts, and our usual shortcasts become instants to throw while moving.
As imbalanced as that may sound at a glance, it would really only add about 30-50 potency and 2-5 mana to each of our usual shortcasts.
Could even call it Chainspell, since it would let us chain our normal longcasts.

Only thing is with Reprise now covering our need for an instant ranged attack, it may not have such high value for us in the latter position.