What overpowered combinations would be available if I allow a bonus action to be used in place of a standard action?

Question

It has come up in game a couple of times that a player might want to cast a spell that has a casting time of 1 bonus action using their "main" action (if they have another bonus action they also want to take on that turn, such as giving bardic inspiration, or controlling a Bigby's hand, etc.)

On the face of it, it seems obvious that something (a bonus action) that is usually much faster than a full action could be done as your full action. Although the question comes up most often with respect to spellcasting, if I house rule this, I would rule that any bonus action can be taken as a regular action instead; however, I would not allow the same type of bonus action to be taken twice (so no giving bardic inspiration to two allies on the same turn, for instance).

Are there any abusive or overpowered combinations I should be wary of if I were to allow a character to take 2 bonus actions instead of one regular action and one bonus action on a turn?

The issue of casting two bonus-action spells would not come up because the rule against casting 2 spells on your turn unless one of them is a cantrip with a casting time of 1 action would still be in effect:

PHB p. 203 (under Bonus Action casting time)

You can't cast another spell during the same turn, except for a cantrip with a casting time of 1 action.

I know it's hard to prove/justify a negative answer to a question like this, but I'd be happy to get answers that say you don't think there would be any issues if you describe how you came to that conclusion.

Answer 1

I'm not sure how unbalancing this would be, but I accidentally allowed this once with Spiritual Weapon. The spell normally uses a bonus action to cast (and attack on the same turn as the casting) and also a bonus action to attack (but it doesn't specify "each subsequent turn"). So, I allowed the cleric to cast the spell as a standard action and attack with it as a bonus action, resulting in two attacks.

Those two attacks were significantly stronger than other characters in the same combat were dishing out (at 3rd level). So, it was somewhat unbalanced, but not overpoweringly so. At higher levels, I think it would balance out.

Specifically, compared to a fighter wielding two weapons (without feats), the cleric got to add casting stat mod to the bonus attack, and neither attack was with a "light" weapon, so I'd say it added about 5 hp of damage.

Answer 2

Reporting back on our experience: Allowing this works fine

I house ruled as follows:

Using a Bonus Action as your regular Action

You may "demote" a bonus action and do it during your regular action. You may not use the same type of bonus action twice in the same round. The rule against casting two spells in a turn (unless one is a cantrip) still stands. I reserve the right to add exceptions if you guys find some way to abuse/exploit/break this.

Exceptions: If you cast Spiritual Weapon using your main action instead of a bonus action, you may not use your bonus action that turn to move and/or re-attack with the weapon.

This has now been play-tested through 25 four-hour sessions of play progressing from levels 1 through 7 with no problems or any additional cases I found I needed to make exceptions for. The PC who uses this the most is the Paladin/Barbarian/Druid multiclass character. I also have a Barbarian/Battle Master, a Warlock, and an Arcane Trickster at the table.

Answer 3

I don't think there will be any balance issues.

The most potent direct-damage (and thus easiest to quantify in terms of abuse potential) tactic I can see resulting from this house-rule is by using an 8th level Eldritch Knight's Action Surge.

He could trade a standard action to cast Magic Weapon as a bonus action, spend his other standard action to cast the melee attack cantrip Shocking Grasp, and then finally make a melee attack as a bonus action with War Magic.

It's an effective opening attack routine but not particularly overpowered. Assuming he's maxed out his Strength by 8th level (he will get three ability score improvements and likely begin play with 16 Strength for a starting bonus of +3), there are two likely possibilities:

1. Great Weapon Fighting Style with a 2-handed 2d6 weapon.
   • [GWF(2d6)+6]+2d8 = 23.33 damage on average.
2. Duelist Fighting Style with a 1-handed 1d8 weapon.
   • [1d8+8]+2d8 = 21.5 damage on average.

Modest damage for that level, and he can only pull off the combination at most twice per long rest. Though, if he can manage to not lose his concentration, he can potentially make Magic Weapon last for up to an hour so again it's an effective opener that buffs his followup attacks while dealing decent but not earth shattering damage. For maximum effect he might spend one of those ability score improvements on the War Caster feat to get advantage on the concentration checks.

Based on this analysis, I'd say your proposed house-rule is fairly safe balance-wise. Odds are pretty good that the best strategies to exploit this will involve Action Surge and/or Metamagic shenanigans, so watch the folks dipping into Fighter and Sorcerer.