that would give players too much advantage. Stacking ton of buffs before battles is a thing in other rpg but i cant see its a thing for Divinity Original Sin , it would be unfair for the Al.
You can already do it in the current game because majority of the buffs last 3 turns except for a few really powerful one that you will use last to minimize the loss. It's just extremely obnoxious to time when one spell end to immediately use the next one to minimize the overall duration loss while also taking way longer than it should to buff before a fight.
There's also cases like where I want to clear the environment of the poison or oils but I can't use battlestomp while the other character is using Fireball.
And it's one of the reason I don't bother having multiple mages atm. I wanted to do something like "2 mages work together, one uses fossil strike and another one uses fireball at the start and it both clashes into each other at the start of the fight for massive explosion" but teamwork is not allowed out-of-combat.