Can True Polymorph create creatures with the Spellcasting feature?

Question

One notable limitation of the Shapechange spell reads as follows:

You transform into an average example of that creature, one without any class levels or the Spellcasting trait.

However, True Polymorph states no such limitation. Does that mean that you could turn into a creature with the Spellcasting feature? Some examples of such creatures would be the Drow Matron Mother, Archmage, Archdruid, etc.

If this is permitted, does it mean you could make creatures with the Spellcasting feature that don't normally have it, provided that you stay within the CR restrictions? For example, could you create a Nothic with Wizard levels?

Answer 1

Yes, you can true polymorph a creature into a spell casting NPC

The rules text is pretty clear about this.

You transform the creature into a different creature ... The transformation lasts for the duration, or until the target drops to 0 hit points or dies. If you concentrate on this spell for the full duration, the transformation lasts until it is dispelled.

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.
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.

If you want to True Polymorph a level 12 or CR 12 creature into an Archmage NPC, CR 12, you can do it, and it will be able to cast its spells.

Archmage (SRD p. 395)
Medium humanoid (any race), any alignment
STR 10 (+0) DEX 14 (+2) CON 12 (+1) INT 20 (+5) WIS 15 (+2) CHA 16 (+3)
Challenge 12
Spellcasting. The archmage is an 18th ‐level spellcaster. Its spellcasting ability is Intelligence (spell save DC 17, +9 to hit with spell attacks). The archmage can cast disguise self and invisibility at will, and has the following wizard spells prepared:

{Bolding mine, spell list edited out for brevity}

For example, could you create a Nothic with Wizard levels?

See this answer for a detailed breakdown of what's in a creature's stat block.

If you look at the Nothic's stat block, you don't find the spell casting ability, so that CR 2 creature would come as is - without spell casting.