Hrm... Let me see how to greatly alter how a class works with just 5 abilities... Let's see... Let's pick "Thaumaturge becomes Red Mage"

Let's see about basing it around being a mix of melee and magical DD, generates TP with magic, combo finisher is a TP move. Base THM, subbing CNJ and GLA.

He has a Refresh spell, self-target only, has to be good enough to sustain somewhat decent MP recovery in active mode to continue bombing with spells. Has a mid-tier spell that generates TP and combos into the melee finisher that generates MP. That's 3 abilities. Also has a melee move that deals some powerful DoT and combos into a spell finisher that drains HP innately (boosted even further by Necrogenesis)

Far more sustainable than Black Mage without endangering himself with Convert, continues to nuke with Thunder line, self-heals, can support with Cure/Stoneskin/Protect/Raise, can throw AoE Blood Rite.

From heavy spike damage to sustained DD/support.

I'm not a pro game designer so i'm very sure I didn't get exactly all the best ideas, but it's a start. What do you guys think that could be done in other cases?