Should be a new mechanic, we don't really need to use any of the current upgrade slots. Can be "1 Squad card max per list". The effects could be anything, and there probably should be a designated leader, but would be nice if it affected all of the ships that meet certain conditions. Generics really need the boost! Shouldn't be mandatory, of course, just a way to make more types of lists viable.
Of course, these things would be released slowly. So could start with just like T-65 and TIE Fighters one (Red and Black squads?) and see how it goes.