There's also the question of a variable-geometry federalism. There are certain elements of cooperation or federalism that all European Union member-states share in--voters in all EU member-states elect people to the European Parliament, to name only a single example--but certain forms of cooperation are only entered into by some countries. Only 17 of the 27 member-states use the Euro and belong to the Eurosystem, for instance, while Ireland and the United Kingdom are both non-members (and Romania and Bulgaria prospective members) of the
Schengen Area allowing for passport-free travel. (Curiously, non-EU Norway and Switzerland
are members.)
Well, as I understand it, the Schengen Agreement actually pre-dates the European Union; it was an agreement between sovereign states, independent of what was then called the European Economic Community, though its members were EEC members, too. It was later grandfathered into E.U. law, but it remains for those non-E.U. members a treaty between sovereign states.
But, yeah, the question of "variable-geometry federalism" is an interesting one. The United States can be argued to practice it today--the residents of the District of Columbia get three votes in the Electoral College, but they get no United States Senators and only a non-voting Delegate to the House of Representatives. The territories -- Puerto Rico, Guam, American Samoa, the U.S. Virgin Islands, and the Northern Mariana Islands -- don't even get to vote in the Electoral College. And for D.C., the Congress must approve and can overrule any Act passed by the Council of the District of Columbia.
We know of at least one form of "variable-geometry federalism" within the UFP -- the Federation Councillors from the five founding Members (Earth, Vulcan, Andor, Tellar, and Alpha Centauri) are always automatically placed on the Federation Security Council, no Presidential nomination or ratification from the full Council required. And we also know that it wasn't until circa 2230 that a Federate from a world outside of the five founding Members, Madza Bral of Trill, was elected President. Perhaps in the wake of Andor's secession, that surviving 22nd Century quirk has been abolished and the five founding wolds no longer get that special privilege? I would certainly hope so.
We know that there are distinctions between
Federation members and
Federation protectorates. Are there categories of associate UFP membership? Are there Euro or Schengen analogues (intensified, optional cooperation) inside the UFP? Et cetera.
Well, a protectorate isn't a member of any kind; it is, rather, a description of a foreign state with a certain kind of relationship with the Federation (or the larger power). It's still a foreign state, and still sovereign, but it is one which the Federation has agreed (with their consent) to defend.
We do know that, even though the Federation possesses all of the powers and authorities that define a sovereign state in its own right, its Members do retain greater autonomy than tends to exist in modern federal systems. We know from
Articles of the Federation, for instance, that Federation Members can conduct their own trade with non-Federation states (such as Nasat's trading relationship with the Tzenkethi Coalition) and can conduct limited forms of foreign relations with non-Federation states (such as Delta's attempting to negotiate with Carrea for their water reclamation system, which was eventually taken over by the Federation government directly). We also know from
Paths of Disharmony that Federation Members retain Ambassadors to the UFP, independent of their Federation Councillors; that Federation Members retain their own Embassies on Earth, where the staffs or residences of their Federation Councillors can be housed (separate from their offices in the Palais) (established in
Bajor: Fragments and Omens); and that Federation Members retain ambassadors to one-another (as Sarek was Vulcan Ambassador to Earth, established in ST09).
Given the Federation government's legal authority to assume control of the Delta-Carrea negotiations, I think the preponderance of evidence is that Federation Members retain limited foreign relations authorities under supervisory guidelines established by the Federation Department of the Exterior. I would also hypothesize that Federation Members can autonomously conduct certain relations and agreements with one-another, again subject to Federation authority (perhaps under the supervision of a Federation Department of the Interior?), perhaps in the same way that
interstate compacts like the
Great Lakes Compact can be concluded among U.S. states and the U.S. government, but on a much larger scale.
I don't see a significant number of Romulan worlds joining the Confederacy of Vulcan. It's not obvious to me that even friendliness towards Vulcan culture would translate into a desire for annexation into the Vulcan state, or that citizens of the Confederacy of Vulcan would like their state's political centre of gravity to be shifted radically to distant expanses colonized by a splinter culture that was traditionally hostile to the homeworld and has a culture radically different from that of the homeworld. (Think of European coldness towards the idea of further expansion of the EU, and then imagine that future prospects weren't limited to the western Balkans and Turkey but Ukraine, Belarus, Russia, and the rest of the former Soviet bloc.)
An excellent point. There again, I would suggest that it's possible that at least a few worlds might want to join the Confederacy of Vulcan -- perhaps not enough to truly shift the center of political gravity within the Vulcan polity, but maybe smaller worlds, more marginalized peripheral Romulan worlds where Unification became popular.
I rather see the Romulan successor state(s) opting for affiliation the Federation in its own right. Especially if it's the less-developed fringes of Romulan space which survive, Romulan civilization is going to need help to survive and continue to develop. How close would it be, though? Romulan membership in the UFP would make the UFP's near-peer enemy of longest standing easily one of the most important member-states of the Federation, quite possibly surpassing United Earth and the Confederacy of Vulcan. What sort of integration scheme would be possible? Would the Federation be able to handle this?
I think that would depend entirely on the specifics of the Romulan state or states that survive after Hobus. I doubt that any Romulan state seeking Federation Membership after Hobus would itself be powerful enough to "outweigh" Earth or Vulcan within the Federation -- and indeed, the Federation Council only gives a single vote to each Federation Member, irrelevant of population. But after Hobus, I suspect that any Romulan state seeking Federation Membership would itself have been rendered so much less powerful as to itself likely equal, but not meaningfully exceed, the relative economic and/or military powers of the majority of Federation Members.
I also suspect that this scenario would occur in a larger context of Romulan disharmony. We'd probably see at least a few Romulan states develop, if there's a Romulan state seeking Membership; the others would likely either strike a nationalist course, or would seek greater integration with the Typhon Pact allies.
Subject species like the Kevratans or Meridians will complicate things. If the RSE's hold over their worlds broke, presumably they would strive for independence and some sort of Federation affiliation to preserve their independence, analogous to the relationships of Drema IV and Bajor after those world's liberations. A close Federation-Romulan alliance, never mind Romulan membership in the Federation, would require radical liberalization of policies towards the Romulan subject species.
Perhaps, but Hobus itself may take care of that. If the surviving Romulan states are busy spending their resources and military capacities just trying to keep themselves together, then expending time and resources on keeping conquered worlds under the Romulan thumb may lose most political support. Cardassia wasn't much interested in reconquering Bajo after losing billions of its citizens to the Dominion -- Hobus may have a similar effect on the surviving Romulan state(s).
I think we can probably take it for granted that while the Federation may tolerate its foreign allies oppressing other cultures within their territory, it would not allow as a Member a state which wishes to continue oppressing a conquered world.
Complicating things further, what of the Typhon Pact? What form might Vulcan-Romulan reunification, or reconciliation, take in the context of the affiliations of the two different branches of the Vulcan species with two mutually-hostile blocs, especially since the Vulcans and Romulans are each major players in these blocs? The alternatives that Spock offered--generalized detente, mutual secession to form a new power bloc--remain valid. (Devastating the RSE is going to hurt the Typhon Pact's overall standing, but it might also empower other member-states.)
As I said, my suspicion is that we'd see multiple Romulan states in the wake of Hobus. One would probably seek greater cooperation with the Pact, to shore up their power base and help rebuild quickly without feeling as though they're indebted to the UFP. If Spock's Unification movement becomes popular enough, we might see another Romulan state, roughly as powerful as the individual Federation Members, seeking Membership for itself. Or we may see some rogue worlds just wishing to join the Confederacy of Vulcan.
Another interesting complication:
We know from ST09 that Spock promised to save Romulus and attempted to do so by himself (apparently without help from Starfleet). Why did he not seek help from Starfleet? Was Starfleet unable to help? We know that his failure to save Romulus led at least some Romulans, like Nero, to blame Spock and the Federation for the Hobus disaster -- but where was the Typhon Pact in all this? Why did the Pact worlds not realize what was happening and try to save Romulus? Are there Romulans who blame the other Pact states the same way Nero blamed Vulcan and the Federation?
), and expanding at multiwarp speeds. Spock's line that the supernova threatening to destroy the galaxy is taken seriously, and backed up with talk of deadly radiation. 
