In the preface to Sets and Classes by Muller, several research programs are outlined that were in development concurrently with publication (or finished slightly beforehand) that he would have liked to discuss, but was unable to for various reasons.
One such program is called ultimate classes, apparently an attempt to explore the outer limits of extensions of $ZF$ via axiomatic projection schemata by adding a new primitive concept to set theory (in addition to $\in$) corresponding to embeddings $\mathbb{V}\to\mathbb{V}$ of the universe into itself.
Apparently, if consistent these schemata provide for the existence of extendible cardinals of high degree in addition to many other large cardinals.
Where can I read about Reinhardt's work on this approach?
Any relevant pointers are greatly appreciated.
In response to a comment by Matt F., here is the relevant preface from Muller's 1976 book.