The spell states that you can not use any of your equipment while transformed, referring to the equipment you would have been wearing while transmuting, so it should be possible to simply lay it on the ground before casting the spell and then just pick it or anything else up afterwards correct?

I'm sorry, I was reading comments to see what the community thought of various uses of this spell, when I saw yours.  It is causing me intense stress.  It is "for all intents and purposes."

The DM could choose to modify or home-brew an item or spell to cause a permanent True Polymorph

Quote from GothGeekGaming >>

I think the  permanent part is only for Object into Creature. That said, a DM can call anything they want in their game.

 It is for all of the transformations.

This combined with simulacrum is just ridiculously overpowered. Create a simulacrum of you at level 20, then turn it into a nightwalker or pit fiend.

Quote from HailRobonia >>

change a boat into a hydra! A wagon into a fire giant! A treasure chest into a mimic!

 The treasure chest might already be a mimic. Fun twist: as you attempt to transform the treasure chest into a mimic, something goes wrong and nothing happens. Upon closer inspection (aka poking), the treasure chest is revealed to actually be a mimic.

So I'd like to play Simic Hybrid Warlock, use this spell to permanently transform into Ancient brass dragon and use the change shape ability to turn back into myself. What happens to my Animal Enhancement and can I still cast my spells and use anything I could have used before like proficiency, feats,
background feature and so on?

I think it’s up to your DM, ultimately. RAW, it says your statistics would be replaced by the creature’s. Then, a dragon’s change shape action says they transform into a beast or humanoid and, as far as I know, Simic Hybrid’s are humanoid so that works fine. The most important part of a dragon’s change shape is the last sentence: “Its statistics and capabilities are otherwise replaced by those of the new form, except any class features or legendary actions of that form.“ So, it is up to your DM to decide what this spell means when it refers to your “statistics” and whether that includes your class features.

To the side of the argument that would support having your class features, this spell states: “The creature is limited in the actions it can perform by the nature of its new form, and it can't speak, cast spells, or take any other action that requires hands or speech, unless its new form is capable of such actions.” This to me says that you would retain your ability to cast spells and any other ability from your class because dragons are known to be able to become spellcasters.

Tldr; If I was your DM, I would absolutely allow it.

Quote from Dragonslayer9 >>

This combined with simulacrum is just ridiculously overpowered. Create a simulacrum of you at level 20, then turn it into a nightwalker or pit fiend.

 Get this, chain simulacrum creation via wish spells and then you have infinite pit fiends. Ludicrously overpowered. Have fun!

thanks for the help

yes

you're wrong, it specifically says "The spell lasts for the duration, or until the target drops to 0 hit points or dies."

So technically speaking if you wanted to you could true polymorph a tarrasque into Tiamat if you really wanted to 

Could this theoretically turn one party member into a clone of another for a time?

What happen if you cast the spell on a non willing creature? Say I turn the BBEG into a snail or a toad, will this work? And if I kill them in this duration, do they stay dead? 

Quote from BoneWhale900 >>

What happen if you cast the spell on a non willing creature? Say I turn the BBEG into a snail or a toad, will this work? And if I kill them in this duration, do they stay dead? 

 If they fail the save, it will work, but if you kill them in their new form they will return to normal.

However, you could turn them into a Brick or something, then maintain concentration for the full duration so they stay that way until the Spell is dispelled, and just bury them in the dirt or something.

With a wizard, can I True Polymorph myself into say; a Pit Fiend, and then afterwards Shapechange into the Pit Fiend and then mix and match traits?

If you choose a dragon that has change shape and assume your own form? Are you still a dragon?

Quote from Nereze >>

Whhhhhhhhy isn't this available for Sorcerers.

 It is tho :)

Quote from Caladrel >>

As has been said countless times on here, you cannot turn yourself into a version of yourself with spellcaster levels through a dragon's innate shapechanging ability. You cannot have the cake and eat it too. Not that easily. 

What you CAN do, though, is true polymorph into said dragon, level yourself up to 20 in a class of your choice (I would just stick to the one I was), and now you have both the physical body of a dragon and the mental state of a level 20 character. You can establish a lair and use legendary/lair actions in your new form, assuming the dungeon master allows you to gain those abilities (not RAW), plus you are a level 20 character on top of that. 

"But grinding back to level 20 will be sooo hard"

Really? You're a CR 20 dragon, meant to be a challenge to a small party of level 20 characters. Go kill an entire village of goblins or something and jump a ton. But since the CR isn't your level, you can still bump your level up until the 20 cap. Congratulations, you are now a CR 20-whatever (again, assuming you can and do boost that) body PLUS a level 20 character. 

Mix that with some other fun permanent things, like the soul jar + true polymorph method to become proficient in all skills, and you're just kicking butt. And that's all completely RAW, possibly RAI (you can start as a weak monster that can gain levels, so why can't a level 17-20 character restart as a cool monster that can gain levels?), and totally possibly in a somewhat quick amount of time. And even if you slow down leveling up in your new form and end up a few levels behind your party, just remember your class levels are all icing on the cake because you're STILL A CR 20 DRAGON. 

Thanks for coming to my TED talk. Next time we'll talk about exploiting simulacra for infinite spell slots, for free. Remember folks, you can take Wish and True Polymorph both as soon as you get 9th level spells!

(One last note, while it's not RAW, your CR might increase from 20 if you are not only a CR 20 monster but also a level 20 character. You could propose to your DM (which is completely up to them because it's not RAW) that your CR goes above 20 and you can True Polymorph into even stronger creatures, then start the process over again.)

 YES