There's a pretty interesting thread by
@DogeEnricoDandolo on the civ wiki (which I'll link
here), suggesting that the Genovese were a planned civ even before the Aztecs but got scrapped for whatever reason. Italy would be cool, but on my money, Rome is way more notable and they'd probably avoid adding two civs with the same capital, except, obviously, Byzantium and the Ottomans. If we only ever get Italian Peninsula merchant republics, that'd be fine representation in my opinion.
That aside, the ones that seem to be leading the pack are any civs that didn't return from 5 and the Berbers, which fill the same role as Morocco. That and the Iroquois are the only ones I'd really be set on seeing return. My other wanted civs would be Hawaii and Ireland.