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?
Best Answer
Here is a list:
I was about to post a related question to this. My suggestion to balance it out is to somehow allow a mechanic whereby, you can spend a number of sorcery points equal to the level of the spell you want to cast. I'm not exactly sure on the balance there, either.
A related but tangential concern is raised by @Alexis Wilke, asking "what's the big deal?" That is, what's the difference between one Wizard casting two fireballs and two Wizards casting one fireball each?
The answer is: a huge difference. If one caster casting two fireball spells is acceptable, then two casters casting one fireball each is suboptimal. Between the two of them, they should cast four fireball spells. You're using a single person's resources in the first case, which is more efficient/powerful than using two people's resources to achieve the same effect.
@Gus raises another good point about the spell slots being limited, which acts as a natural stopper for this. This is true, and it seems to be what is preventing the querent's party mates from spamming all their spells this way.
For small fights, it's no issue; and by "small" I mean fights that the DM meant to be easily winnable. But for boss fights, this becomes problematic. You can bet your players will want to be well-rested before they run into the BBEG's secret lair, and showering them with two high damage spells per caster per round may make that encounter trivial and anti-climactic.