-give all monster 2 elemental resistances and 2 weakness (1 of each for WHM and the other for BLM). resistance -10%dmg, weakness +10dmg.
-turn surecast, manaward and aetherial manipulation into, fire, ice and thunder stances.
-make these stances low level skills.
-taking part of the idea in the op, make every stance switch how element works:
-- in thunder stance: thunder element would be the nuke, the non weakness non resistance element the dot, and the resisted element the MP recovery spells (resistances and weakness must be fixed to not break this rotation)(rotation and skill list would be practically the same)
- to give more utility to BLM, turn manawall into a trigger skill, lets name it "elemental aura", once one nuke rotation have been completed apply a elemental charge(buff) of ice, thunder or fire . upon applying the previously mentioned trigger ability (elemental aura) this elemental charge will be converted into a party buff (elemental aura (5 minutes)), fire: magic attack and physical attack+3%, ice: physical defense +3%, thunder: magical defense +3%, only one can be applied(the rotation would be changed from one stance to another only to apply this minor convenience as needed)
-whm as healer role can keep the same skills, the elemental wheel will keep the dmg balanced by itself.
- keep ligth and dark elements neutral without weakness nor resistances.
This is just one idea, not what they should so, because in favor of convenience the best decision would be leaving BLM as is to not disturb the player base who is already used to it, and applying the elemental wheel only to another job like geomancer or elemnetalist.
This addition could bring elemental weapons to the game, which could give a very small additional damage to auto attack (separated from normal damage) but not be a disadvantage when resisted, adding more flavor tho the game.