What would be the repercussions of an immediate Catalan EU bid?
That was discussed during the Scottish independence referendum if I remember correctly
The problem is that an immediate bid will be met by an immediate veto from the Spanish national governnent. There is no situation that allows for a quick-fix trade agreement really. Not one that is politically feasible, anyway. The Germans and the French aren't interested in having more microstates in the EU, particularly when it destabilizes core members of the EU like Spain or Italy.
The thing is, we aren't more important than UK, but we are key in Spain, Catalonia will have to undergo a huge crisis for years if we go independence? Yeah, but Spain as well, with a debt of already 100%+ of the GDP, Spain will fall into his worst recesion ever, shaking the grounds of the UE, basically a several times worse Greece crisis.
Do you really think Spain is going to let Catalonia walk without taking any of the debt? That alone is going to take years to haggle over and will delay any type of EU accession. That's not even counting the number of social services that will be dramatically cut across Catalonia and Spain while the economy contracts in both.
Look I'm a half-Spaniard living in the US. I am fine with people voting whatever they want but a lot of people are being misled about the consequences of their vote. If folks want Brexit, that's fine, just make sure everyone knows what is going to happen.