Starting at 0 MP could work if MP regen was a bit faster, and then you could make any equipment/stat that currently affects cast speed increase starting MP or reduce MP costs or something. It's an approach that makes the game more dynamic as opposed to losing control of your characters for the duration of the cast time.
I do agree that some spells that are most useful right at the beginning of each battle would have to be addressed individually as to not become unviable or outright useless (and I don't think you'd want to incentivize strats that involve entering a battle and waiting/running around for your mp to fill up before you can cast that first buff/debuff, sounds kind of counter-intuitive.)
Anyways, I recon It'd be a LOT of trouble to make that change now that you probably balanced the game around the current MP system, but I'd be interested in trying it out nonetheless.