Being able to use any amount of spell slots is kinda fun but... it will severely impact balance. Imagine sorcerers with quicken spell and haste. Three fireballs in one round.... o_o
Not being hugely knowledgeable about 5e, I’m not sure how it should work. I agree casting multiple levelled spells as actions seems massively overpowered and would definitely be on board for limiting that. I’m not so fussed about - and in fact like - being able to cast one levelled spell as an action and another as a bonus action (e.g. Thunderwave nearby enemies and Misty Step away) given the spells that use bonus actions don’t tend to be attacking ones. I’m not sure whether even that would be permitted under 5e?
