Some are more appropriate than others to be honest and some can be just amalgams civs that can encompass large periods of time (i.e Russia, Ethiopia, Japan).
I think we can go through this list though quite easily:
Arabian Peninsula: Hosts a myriad of civs that can be split. Modern inclusions are great in the form of Arabia as the Saudis are certainly not in any form of candidacy for being a fleshed out civ.
Nice Splits: Arabia, Assyria, Sumer, Babylon, Nabatea, Persia, Selucids, Greece, Ottomans
English Isles: Scotland is enough worthy of a civ, it has the opportunity to spout a whole lot of unique flavor. As does the Irish. UK wouldn't make sense as an individual civ seeing as England is more of an empire on the fringes of Medieval right through to the 1800s
Nice Splits: Scotland under James IV, England, Ireland under Brian Boru
Spain: Portugal and Spain are easy to split - but the question rather lies whether or not it is appropriate to split Spain up further. The basques are nice and flavorful, as is Castille, Leon and Aragon. Isabella does do a great job at representing a unified and utterly powerful Spanish empire. I would rather elements of each of these minor Spanish civs to pop up as flavor throughout Spains unique attributes or via social policies.
Nice Splits: Portugal and Spain.
France: The big one here is the splitting of the Franks and France. Is it justified? Of course it is. Each is systematically different and privdes flavor thoughout. Other candidates could be a Shoshone-esque (i.e oddball tribe) germanic/gothic tribe such as the Gauls. The Swiss are also nice as more of a flavorful inclusion as a touch of salt rather than a fleshed out empire.
Nice Splits: Franks under Charlemagne, France under Napoleon, Oddball barb tribe, Switzerland
Germany: Germany is really nice and has the opportunity to be split up a thousand times over. Obviously the big one here is the HRE and Germany. HRE, although not lasting all too long was an amazing political institution with considerable influence unlike the world has seen before. It'd be nice in a scenario though. The HRE I feel would be better replicated in the form of a complex game mechanic or a social policy which turns things upside down as they did in real life. Currently I feel the HRE is already represented in Germany via the Hanse (as are the Germanic tribes and the Nazis in the UA).
Nice Splits: Bohemia, Austria, Germany
Italy: Rome and Italy in my personal opinion are warranted a split alongside Venice. Rome is fantastic as a Classical era powerhouse - but the references to Italy stop there. 3 Major Renaissance Italian states stick out. Venice is an obvious one. Less obvious are the Papal States. JFD has a mod which other than the leaderscreen looks and feels as if it slides into Civ V absolutely perfectly and thus proves it's worthiness for it's inclusion in civ. The third one is the one which people fight over: Italy. The argument for not including Italy is in my opinion ridiculous. To say that Italy is represented by the Romans is the same as saying that Mexico as a modern state is represented by the Aztecs. Italy's great states (Sicily, Tuscany, Milan etc.) would be nice individually (if there were just one of them) but would also be nice to represent Italy as a whole. I'd combine the three states into a fully fleshed out Italy civ that represents Italy from the Renaissance to around <1900.
Nice Splits: Rome, Venice, Papal States, Sicily/Tuscany/Milan (mutually exclusive) or as a Ren-1900 Italy civ focused on Golden Ages/Wonders.
I might continue this list further later on if it's wanted.