PIE has been pretty well reconstructed up to the time of the breakup
into Hittite, Indo-Iranian, Proto-Germanic, etc. We also have some
tantalyzing clues to its earlier structure.Perhaps before trying to
link PIE with other language families we should come up with a
reconstruction of Pre-PIE.

Some issues to be addressed:

How many laryngeals and what was their manner of articulation?

PPIE was apparently an ergative language. What was the structure of
nominal morphology?

Was there a feminine gender?

How many vowels were there?

What was the origin of e~o alternation?

What, if any, function did thematic vowels have in nouns and verbs?

How, if at all, does nominal morphology relate to pronomial

What was the function of heteroclitic declension?

Lots and lots of other issues...