Would allowing a cantrip cast as a bonus action to be used on the same turn as a non-cantrip cast as an action break the game?

Question

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?

Answer 1

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.

Answer 2

The house rule only improves two cantrips, and not by much.

The example you provided where a character could cast a spell like magic missile with their action and shillelagh with their bonus action is indeed a benefit to the player, but hardly overpowered or gamebreaking. Since you only get one bonus action per turn, it can take multiple turns to use the different spells and features that take bonus actions.

If we expand on this example and explore differences, it's easy to see the implications of this change. Let's say this character also wants to cast hunter's mark on some target and attack that target with the benefit of both spells. Without this house rule, it would take three turns to cast all three spells and attack.

1. Turn 1 - Action: magic missile
2. Turn 2 - Bonus Action: shillelagh
3. Turn 3 - Bonus Action: hunter's mark, Action: Attack

The house rule allows you to cast all three spells in two turns.

1. Turn 1 - Action: magic missile, Bonus Action: shillelagh
2. Turn 2 - Bonus Action: hunter's mark, Action: Attack

Essentially, you are adding an option for your bonus action where it normally would not be able to be used. This seems powerful but considering that this house rule only applies to cantrips that can be used as bonus actions, and the only spells that can be cast as bonus actions are shillelagh and magic stone, it won't have any serious consequences for the action economy because it only benefits players that have those two cantrips, and would use them in a similar case.

As @Ceribia's answer has already mentioned, this house rule can't even necessarily be exploited using a Sorcerer's Quickened Spell metamagic option. Any time you would quicken a cantrip to attempt to benefit from this house rule, you could just as easily quicken the "normal" spell and achieve the same result.

Answer 3

To fully answer this question, you would likely have to look at the combinations of spells that can be used (I do not believe it is very many). I can think of two immediate issues that could result from this.

However... There are ways to cast spells as a bonus action. Quickened Spell for instance. This may cause some problems assuming a literal interpretation of your wording, since now it means that he could quicken other cantrips as a bonus actions.

Quickened Spell: When you cast a spell that has a casting time of 1 action, you can spend 2 sorcery points to change the casting time to 1 bonus action for this casting.

So for your purposes - in a single turn I could use some sorcery points and cast a cantrip such as Blade Ward as a bonus action, and then cast a regular spell action.

1. If you are fighting an enemy who can use reactions to counter your abilities - he will only ever be able to counter one and be hit by the second.
2. Cantrips that can be used to boost your performance for a turn (that were excluding you from the advantage) can now be cast when needed with no penalty.