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.