Latin has some nouns and adjectives derived from pronouns: unicus, identitas, qualitas, neutralis… I have the impression that such derivations are mainly later than classical, but I do not really know. These derivatives look like something I would expect to be more common in post-classical scientific language than classical prose. When did such derivations appear? Are they already present in classical Latin? Did they become more common at some later point? Partial answers are also welcome, as I have no more understanding than I what have shared here.
You want a partial answer? Very well, ecce:
I believe some of those words were coined in classical times, but others (much) later. I also suspect that some were invented in intellectual discourse, often as direct translations of (equally abstract) Greek words.
qualitates igitur appellavi quas ποιότητας Graeci vocant, quod ipsum apud Graecos non est vulgi verbum sed philosophorum, atque id in multis — Cicero, Academica 1.25.7.
Qualitas seems to have been fairly common from Cicero onwards.
Libanus: Sicut tuom vis unicum gnatum tuae superesse vitae sospitem et superstitem, ita ted obtestor per senectutem tuam — Plautus, Asinaria 16.
Unicus seems to have been commonly used even in Republican times, such as by Plautus and Lucretius.
Nec statim diligentem putabo qui promiscua, quae epicoena dicuntur, ostenderit, in quibus sexus uterque per alterum apparet, aut quae feminina positione mares aut neutrali feminas significant, qualia sunt 'Murena' et 'Glycerium'. — Quintilianus, Institutio Oratoria 184.108.40.206.
This grammatical term seems to have been in common use during the Principate and later.
I could not find any examples of identitas from classical authors, nor is the word found in Lewis & Short.