A complete guide can address all popular psionic assumption rather than make a choice on a specific assumption on the nature of psionic. and let DM use what is best for their setting.
So we could have both :
the psionic are magic assumption
and the psionic are mundane natural skill improvement assumption.
We can have the completion of psionic subclasses begin in Tasha.
The creation of psionic version of each classes.
The famous psion class that dont use spells but rather a kind of psionic skills and mastery,
And a more general psionic system that can be use on top of character classes like in first Ed.