Braga and Guimaraes have decent support, but the rest could be absorbed into the Spanish lower league system and nobody would bat an eye lid.
Like VI says, there's little chance of the big 3 Portuguese clubs nor the bigger Spanish clubs giving up CL/EL places with the added competition. So without adjustment to expand entrance for a combined league its a non starter. If they ever did though, it would make La Liga ridiculously strong. Porto and Benfica have managed to stay competitive in Europe despite playing in what is a minor league, they'd be powerhouses in La Liga (again so long as the CL/EL places are adjusted).
I honestly think this kind of stuff is necessary for the future of big clubs from smaller leagues. A Northern European league for one but particularly with former USSR and Slavic countries as they don't even have much history as separate leagues to forego. Although there's obvious geopolitical issues there.