nominatives

In the Cippus Perusinus, ipa is certainly in the nominative case matching corresponding nominatives ita 'that' and ica 'this' but the question is whether this pronoun's declined according to its role in the relative clause or its antecedent, tezan, which I give the value of 'cippus':

Relative pronouns in Etruscan 2010

I don't know what your "generalized observation" is but "Latin marks its masculine nominatives with -s" is an incorrect statement by the standards of any Latin handbook.

The etymology of Latin tofus 'tufa' isn't written in stone 2009

Ardagastus: "You say 'Since, unlike Etruscan, Latin marks its masculine nominatives with -s like all faithful PIE languages do, the result of tōfus is unproblematic.' but this is not correct."

The etymology of Latin tofus 'tufa' isn't written in stone 2009

Since, unlike Etruscan, Latin marks its masculine nominatives with -s like all faithful PIE languages do, the result of tōfus is unproblematic.

The etymology of Latin tofus 'tufa' isn't written in stone 2009

You say "Since, unlike Etruscan, Latin marks its masculine nominatives with -s like all faithful PIE languages do, the result of tōfus is unproblematic." but this is not correct.

The etymology of Latin tofus 'tufa' isn't written in stone 2009

"I don't know what your 'generalized observation' is but 'Latin marks its masculine nominatives with -s' is an incorrect statement by the standards of any Latin handbook."

The etymology of Latin tofus 'tufa' isn't written in stone 2009

Ardagastus: "... but 'Latin marks its masculine nominatives with -s' is an incorrect statement by the standards of any Latin handbook."

The etymology of Latin tofus 'tufa' isn't written in stone 2009

This then may explain the later pattern in the PIE system and explains how the nominative came to be marked when the tendency in languages is for nominatives to be unmarked.

The early Indo-European case system and definiteness 2008

I believe that in the midst of the Late IE period, these genitive-derived adjectives became misanalysed as thematic nominatives ending in *-z because this gives us a source of PIE's neuter thematic adjective in *-ó-m i.e. the genitive plural was reanalysed as a neuter thematic.

Rasmussen's consonantal *o and laryngeal deletion 2008

This strong/weak syllable thing may seem crazy to you but if you think about it, it's a great way to distinguish nominatives from genitive singulars when they end up with similar endings: eLIE thematic nominative singular *-əz versus nominative plural *-es versus genitive *-ós.

Rasmussen's consonantal *o and laryngeal deletion 2008