Finding the Hacks That Allow Multiple Leveled Spells Per Turn

 The rules for spellcasting in D&D are changing.

In the 2014 rules, you might be surprised to find out that the actual limitations on how many leveled spells you could cast on a turn were really baked in specifically to spells with a casting time of a bonus action.

But the new rules are simpler: you can only cast one spell that uses a spell slot on each of your turns. The rest of the action economy is basically taken care of because you only have one reaction, though it's important to note that this will, in fact, prevent the old countering-the-counterspell mage battles of yore. Indeed, if you try to cast a spell and then run away from a foe, you won't be able to cast Shield if they hit you with an opportunity attack.

Now, cantrips are, of course, still an exception here. You can still cast Hex and Eldritch Blast on your opening turn as a Warlock.

However, because the restriction is not "a spell of 1st level or higher," but rather "a spell that costs a spell slot," there are actually all manner of shenanigans that might come up.

Many subclasses (and at least one class) have means of casting spells without using spell slots. Paladins' Divine Smite is now both a spell and a bonus action to cast, but they do get one free use of it per day - which means that if you have a reaction spell you need to use on your turn, you can do so.

Likewise, the newly-redesigned Archfey Warlock will be able to cast Misty Step several times for free each day. Because this isn't taking a spell slot, you can now bamf out of danger and then maybe leave a Summoned Fey in your place.

I think one of the biggest winners here will be the Aberrant Sorcerer. Sorcerers, of course, are a class that is hit particularly hard by the old spell restrictions. But thanks to Psionic Sorcery, not only do they get to cast spells without any components, but they are also using Sorcery Points to do so (in fact, given that you can convert a spell slot you'd need to cast the spell regularly into the same number of sorcery points you're spending, I think unless you desperately need to keep your bonus action that turn, you should pretty much always cast Psionic Spells in this manner). This means, then, that you can Quicken any other spell you like to cast alongside the Psionic Spell. Pop out those Evard's Black Tentacles to restrain your foes, and then quicken a Fireball to drop on top of them!