In my group I was planning on making some house rule changes to the section on bonus action spells, as I'm not a fan of how it's currently written. I'd prefer the normal action(spell) + bonus action(cantrip) combination to be valid:
If you use your bonus action to cast a spell, you can't cast another spell during the same turn, except for a cantrip with a casting time of 1 action.
Instead, I was planning on having it be:
If you use your bonus action to cast a non-cantrip spell, you can't cast another spell during the same turn, except for a cantrip with a casting time of 1 action.
For example under the normal rules a caster could not cast Shillelagh (a bonus action spell) on the same turn as Magic Missile (a normal action spell). Under my house-rule the character would be allowed to since Shillelagh is a cantrip.
This feels more streamlined to me, but I'm wary of unintended consequences. Would wording it this way break the game in a way I'm not thinking of?
Best Answer
This house rule really only changes two not particularly powerful spells and is unlikely to unbalance anything
This house rule would only change anything when the character casts a bonus action cantrip spell. Searching on donjon I can only find two such spells, shillelagh and magic stone. Both spells give the caster a weapon to use so without spending the casters action to use it they aren't getting much out of the spell. Based on how limited the spells affected are I don't believe this change would unbalance anything or even come up that often.
Quickening doesn't change anything since the cost to quicken a non-cantrip spell is the same
As @MrNattious pointed out the cost to quicken a non-cantrip spell is the same as the cost to quicken a cantrip. Under the normal rule you could cast any full-action cantrip and whatever quickened spell (be it cantrip or non-cantrip) you wanted along side it. With this house rule you could quicken the full-action cantrip and cast the other spell a a full-action to get the same result. There is no change.