Learn English – the antonym of the prefix retro-

Meta- comes from Ancient Greek, and in this sense means beyond, with, about or after. An example is Aristotle's Metaphysics which his editors placed after his Physics, and started readers thinking it might mean more than location. That suggests to me that the Latin pre- or Greek pro- could be possible antonyms used in a similar way.

But this would only help if people used it. Shall we try to push premetadata or prometadata? Or would actual data be more easily understood?

The Online Etymology Dictionary reports the following:

early 13c., amasian "stupefy, make crazy," from a-, probably used here as an intensive prefix, + -masian, related to maze (q.v.). Sense of "overwhelm with wonder" is from 1580s.

So the a- is a prefix here, but it intensifies instead of negating. Wiktionary lists the several uses of a- and notes the Greek-based meaning you cite as the only one that remains productive today.