As for Class Features, I think that's almost entirely the purview of custom NPCs that the DM has added class levels onto and Sidekicks. There are a few particular NPCs like Warlock of the Archfey whose innate spellcasting features could be said to be Warlock Eldritch Invocations or the like, though, but apparently even the Spellcasting trait in NPC statblocks is no longer a class feature since the wording got changed after the first printing.

If you have True Polymorph you can also technically take a creature like a dragon, turn it into a humanoid, Magic Jar into it, and then stop concentrating. Naturally, what happens next is up to the particular DM's ruling, but it is worth consideration and discussion to find out what that ruling is (preferably in advance rather than during actual play). Another thing True Polymorph can do is give you access to the curse of lycanthropy and thus the potential to increase the CR of a given humanoid you intend to possess or at least give it immunity to non-magical weapons that aren't silvered. Although at level 17+, I'm not sure how many weapon-using foes will not have their attacks count as magical. Especially since WOTC reportedly is moving in the direction of just making enemies deal Force damage in order to get around resistances or simplify in their minds the difference between an attack with a magic qeapon and a non-magical weapon.

Step 1: be a blade singer and hire a hardy yet nimble mercenary. (High CON + DEX). Step 2: in the fine print of the contract, he must provide 1cubic inch for a clone which the party maintains possession of if not used before the contract ends. Step 3: when the clone is matured and the contract ends, use magic jar to transfer your soul into the souless clone keeping your INT, WIS, and CHA; but gaining the mercs STR, DEX, and CON. Step 4: show up to the mercs home claiming to be his long lost twin brother.

Step 1. Commission a medium sized (lifelike... maybe 6 inches taller than yourself?) hollow statue worth at least 500 gp. Step 2. Cast Magic Jar and have your soul go into the 500gp statue. Step 3. Have your Simulacrum (make sure to have already cast your Simulacrum) cast True Polymorph to change an object into a creature. The size cannot exceed the object's size, this is why you picked a medium statue. And it's CR must be 9 or lower (my current fav is a Inquisitor of the Sword, medium humanoid CR 8, it's from the van richten's book). Step 4. Have your soul take over the Inquisitor of the Sword (not sure if your soul goes anywhere as what you used for the Magic Jar is also the target you are now possessing?)(or does the object that was changed into a creature have a soul?). Step 5. Cast Clone and make a clone of the Inquisitor's body (that you now possess). However you end the Magic Jar and True Polymorph, so long as you are still alive and able to go back to your original body, is all that matters. Once your clone matures (120 days I think) you can do something reckless that results in your death and your soul will go into the clone... which is a way to permanently get your soul into the Inquisitor of the Sword. You can do this with any creature if you juggle the right spells at the right time. Keep in mind that Magic Jar only targets humanoids.

I would love to be able to use this with the clone spell. The closest to doing that RAW is going to be using Simulacrum to replicate a particular body and then possessing it as a disposable ice puppet.

Here is the trick: This is a player accessible budget phylactery. It has 3 main uses: 1 possess the strongest, beefiest (maybe a level 20 barbarian) humanoid you can find, wizards have a d6 hit die, and most player races don't have that much spice. Magic jar allows for getting all those hit points and physical stats. And when it is a monster and not a PC you nabbed, you also get the multiattack, maybe a legendary resistance or action, its all fair game. 2 Get in anywhere, just nab the closest enemy (and let your party carry you in the meantime) If you are about to die, hold your action to swap back, then swap just before the killing blow lands. Afterword's, posses something new and try again, if you are some random gemstone it should be relatively fine. 3 Somehow find a shapeshifting dragon (ancient gold would do), convince it to transform into a humanoid, steal its soul, swallow the magic jar, and now you are an ancient dragon wizard, having the best of both worlds.

Step 1: Magic Jar Step 2: Create Magen Step 3: ????? Step 4: Profit