Yes.  Yes to all of that. 

That's the reason I dropped in the "Potentially" on there. 

There's always that 1-in-a-million.

I think, if you have to use a 9th level spell before you find something "game-breaking", that the rule is fine.  9th level spells break the game by existing.

And, if you use the already proposed "Can't trigger the same effect twice in a single turn" restriction, this one potential bit of breakage can't even happen anyway.

Someone already brought up an example - the Cleric with both Flame Sphere and Spirtual Weapon up - where two different effects both require a Bonus Action to trigger.  I think if the PC in this situation wants to focus their full attention on *using* these two legally cast spells, it's probably fine.

In a previous session, my sorcerer had a Giant Mountain Goat from a bag of tricks, and had cast a storm sphere.  All I wanted to do was use the once/round storm blast (Bonus Action), and command my Goat to defend me (Bonus Action).  Hardly game-breaking, considering I was giving up the option to do a 2d10 firebolt (as an actual Action)...

Err, so what's stopping them from using Blade of Disaster and Meteor Swarm on the same turn? Lmao 😂 there are also ways to change the damage type of meteor swamp to force.

I personally just had a situation where I let the same bonus action be taken twice, but it was also a pretty over the top situation. I had a player who was a multi-classed Monk/fighter, who was under the effect of Haste and used their action surge in the same turn. I allowed them to attack, flurry of blows, extra attack, action surge attack, and use flurry of blows again for their haste action. I feel like it fits a bit more if you have the action that allows the bonus action being used again, such as the case with flurry. It's also a completely different iteration of the same bonus action, rather than moving an aoe to proc it's effect more than once, it's closer to casting a single action spell twice in one turn. I also think Martials need whatever they can get, especially monks without magic weapons.

Quote from Sagidec >>

Err, so what's stopping them from using Blade of Disaster and Meteor Swarm on the same turn? Lmao 😂 there are also ways to change the damage type of meteor swamp to force.

 Quantity of spells slots.

Quote from KristopherTHaven >>

9th Level Spell Blade of Disaster: 

Casting Time. 1 Bonus Action

Concentration. for up to 1 minute.

 2 Melee Spell Attacks. 

EACH does 4d12 Force Damage. EACH crits at 18 or above on the d20 roll.  EACH when they crit does ANOTHER 8d12 Force Damage (for a Total of 12 d12 Force Damage)...  PER STRIKE that Hits and Crits.

On your turn, you can move it up to 30 feet and make 2 melee spell attacks as a Bonus Action. 

That's 288 POTENTIAL points of FORCE Damage. 

Now imagine you can trade your ACTION down to a BONUS ACTION. 

Now you have TWO Bonus Actions. 

Now you can attack with it 4 times...  per turn. 

That is 576 points of FORCE Damage, potentially, per turn.  Force Damage is THE type of damage that is the LEAST guarded or resisted against.  It is THE most devestating type of damage in the game. 

For reliable damage, that absolutely destroys stuff, Blade of Disaster is one of THE most destructive spells in the game.  It works against creatures, structures, and loose items.  Even Meteor Swarm doesn't do that much damage. 

You could very literally One-Shot many of the deities, planar beings, and ancient dragons.  You could literally One-Shot the CR 30 Legendary Dragon monster Aspect of Tiamat.  You can potentially kill TIAMAT...  AND the Tarrasque...  in only TWO...  Rounds. 

That's why.  :-)  Well, one reason why. 

In 4th Edition D&D you could trade an Action down to a Bonus Action. 

It was one of THE worst ideas WotC ever came up with.  

It was one of the PRIMARY reasons why 4th Edition was such a nightmarish failure.  :-)

 Computing max damage and computing the probability of getting it are two very different things, and doing the former without the latter is just ignoring the facts to make an argument. As Farling said, it's a 15% chance of critting on each attack. If my math is right, that means you have a 0.050625% chance of critting four times. And then your chance of rolling max is 1 in 12^48. In total, that's incredibly low - far, far lower than 1 in 1,000,000. As in 8.something * 10^-56%.

Ain't gonna happen. Ever.

However, you are right that making bonus actions and regular actions interchangeable would unbalance the game. Not by quite as much as you're saying it would, but it still could easily create problems.

You can now if your dm allows the spell driver feat

So if I understood what I read then it is the DM´s choice (it always is but still). I just thought kt would be funny that a dhampir mystic could have 300 ft. Climbing speed in 1 turn for 1 turn, BUT you need to bonus actions to do so.

While a late post, there is a clear answer to this question in Sage Advice:

Can a bonus action be used as an action or vice versa? For example, can a bard use a bonus action to grant a Bardic Inspiration die and an action to cast healing word?

No. Actions and bonus actions aren’t interchangeable. In the example, the bard could use Bardic Inspiration or healing word on a turn, not both.

You can't cast two spells on the same turn unless one is quickened with metamagic regardless of standard cast duration

No not without weird game stuff that I don't want to go into or the DM's permission.

Hello, Necromancer here..

You can cast up to four leveled spells in a single turn. Here's how.

https://www.reddit.com/r/mattcolville/s/Ara3tvM1zj

:)

Quote from BlackXIV >>

You can't cast two spells on the same turn unless one is quickened with metamagic regardless of standard cast duration

 Of course you can. Two levels of fighter for Action Surge and you can cast a leveled spell as your action, Action Surge, then cast another leveled spell. Which they are doing away with in 2024 if the UA version stays. 

if you quicken a spell, as you suggest, then your action would have to be a cantrip. 

Quote from BlackXIV >>

You can't cast two spells on the same turn unless one is quickened with metamagic regardless of standard cast duration

 You can cast two spell on the same turn  1 as an action and 1 as a reaction just fine.

Quote from Plaguescarred >>

Quote from BlackXIV >>

You can't cast two spells on the same turn unless one is quickened with metamagic regardless of standard cast duration

 You can cast two spell on the same turn  1 as an action and 1 as a reaction just fine.

 Good point. I always forget about those reaction spells. Keep thinking they would be on another turn but something like counterspell can be the same turn you cast something else.