Leaving aside anecdotic discussions about the governmental interests of both Spain and Catalonia, it is still undefined who has the right to claim sovereignty. The arguments from one side and the other focus on short and medium term impact but even the Catalans seem to be forgotten.
Assuming that both claiming parties, that is Spain and Catalonia finally decided to leave behind egoistical interests and were looking for a real, definitive and peaceful solution for the dispute, before going into any negotiations, they would need to agree on who would be able to participate. Indeed, this is the key question we have to answer first, particularly important to all the claiming parties because Spain and Catalonia opposed to each other.
Any claiming party in any sovereignty conflict should have what I call a “colourable claim.” (Núñez 2017)* In other words, they must have a valid reason to claim sovereignty over Catalonia—obvious examples are effective occupation, consent by the other agent in the dispute, consent by other States, and/or consent by the international community. That reason should be “colourable” enough to prove that their intended rights are at least plausible to be acknowledged, and they can be based on any reasonable circumstances—e.g. political, historical, legal, geographical arguments to name a few. For example, in the case of Catalonia it would be unreasonable for China to participate in the negotiations in relation to their sovereignty since they do not have any colourable claim over that territory.
Some questions may arise, in particular, in relation to the participation of Catalans in the negotiations. On the one hand, would it be fair for them to negotiate the sovereignty of the territory they are inhabitants of? On the other hand, do we need some minimum criteria here to help us determine when the residents of the disputed territory get a vote?
If we pursue a solution both possible and fair, to ask Catalans to go into negotiations about their sovereignty is fair, provided that their right participate is welcomed by all the claiming parties—i.e. they have a colourable claim.