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.
Best Answer
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.