When I say 'Spellcasting Action Economy' I mean the rules that state when you cast a spell (Cantrip or otherwise) as a bonus action during your turn, the only other spell you can cast that same turn is a Cantrip that must have a casting time of 1 action.
There are two classes in the campaign I take part in that utilize Meta Magic with Sorcery Points. Quicken Spell does not give any exception to the spellcasting economy rules outlined above, but allows you to turn a spell with a casting time of 1 action into a casting time of 1 bonus action.
Our DM would like to offer some uniqueness to Quicken Spell, and interpret it as an exception to the rule: allowing those with Quicken Spell to cast their altered spell with a bonus action and then cast a 1 action spell that doesn't have to be a cantrip if they so wish. (Or vice versa: 1 Action spell then Quicken Spell Bonus Action.) While I do not disagree with this addition to Quicken Spell on a RP / Class level, I am curious about its balance.
My question is: What are the possibilities for abuse, or general consequences, of house-ruling only Quicken Spell usage to break the spellcasting action economy instead of using it as written?