The basis for my question is this line in the true polymorph spell (emphasis mine):
Creature into Creature. If you turn a creature into another kind of creature, the new form can be any kind you choose whose challenge rating is equal to or less than the target’s (or its level, if the target doesn’t have a challenge rating). The target’s game statistics, including mental ability scores, are replaced by the statistics of the new form. It retains its alignment and personality.
By RAW, does that mean that you can effectively craft a creature – say, a "PC" with class levels – outside of established Monster Manual entries? (A new form.)
Sidenote: I am aware that a great deal of true polymorph shenanigans are up to DM interpretation. Our server has multiple DMs, so I am seeking evidence-supported answers that we can all fundamentally agree on without straying too much into our own interpretations.
Best Answer
Probably not (except for some minor customization).
As you said, many True Polymorph shenanigans fall under DM fiat. This includes whether you can polymorph into a specific creature (and not a generic example of a specific kind of creature), and others of the sort.
You, the caster, can essentially choose a kind of creature. While the DM has leeway to create new monsters for his world and adventure, players (in every table I've ever been, but again, there can be exceptions) don't usually have the power to just invent a new kind of monster.
I would adjudicate, for your server, that you can only True Polymorph a target into one of three:
an officially existing creature, taken from the MM, VGM, MToF, etc. This includes variant options of monsters (like the Shadow Dragon). All of these should still have the approval of the DM, who may claim a given creature does not exist in his setting
an officially existing NPC, with a specific (DM approved) race. Appendix B of the MM and of VGM describe class-based NPCs (Bards, Warlocks, etc) as any race, and claims the DM can can add racial traits to further customize them. it seems logical to assume that, when True Polymorphing into one, the DM (or player) could logically pick an adequate race.
a homebrewed creature that the DM has created and that the players know exists in the setting
This basically prevents players from being able to make up some creature that fits exactly all the requirements they need for some specific problem, it gives the DM a basis for obtaining statistics for creatures (either from the official sources or from their own notes), and it also gives the players a fixed set of sources (the books and the DM's notes) from which they can draw ideas and kinds of monsters.
Being a server, this should be a proposition that follows your social contract (e.g., all DMs agree on it, or something similar).