I posted this in the OT but for Red Mage speculation...
It seems like spells power up melee, and I wouldn't be surprised if the inverse is true. The spells the trailer shows each have a corresponding melee equivalent if we're looking at the particle effects--which are fairly consistent with casters as it is. The first spell is purple/non-elemental, the second is white/holy. The first melee ability is white/holy, the second is purple/non-elemental. The backflip is fire, the flare-like is fire. So, you cast spells to make the next melee of that element stronger, and you use melee to make the next spell of that element stronger. If you do a melee atack without a charge, it creates one to buff the spell. That sort of thing! Cast a spell without a charge, it'll create one to buff the melee. It creates a neat back-and-forth and I hope they'd keep it to those three elements to give you multiple options.
For example: You can cast holy, use a lunge the expend that holy charge or you can use some other melee ability to expend that charge. This way you aren't stuck having to do the back-flip or lunge if you want to build/expend your holy and flare charges.
Chainspells likely just makes <x> amount of spells instant cast, or all spells are instant cast for <y> amount of time, which allows you to do burst damage by stacking everything all at once. Build all three stacks, expend all three immediately.