Why wait after until the League, you ask? Easy. The League is when Team Plasma really kicked things into gear in BW1. Have Ash be about to win the tournament, then N crashes it and denies him victory (trolololol), and thus begins the TP filler arc. They didn't bother doing anything before the League because their debut was being saved for it, and their plot was being saved for after that. As for the subject of the incentive to promote Unova games heavily decreasing, one can already ask why they intend to promote Genesect alongside a Gen 6 Pokemon rather than simply shooing it away like Meloetta and focusing solely on the new Pokemon.The writers have had ample time to do something with Team Plasma, but they didn't use it. Why wait until after the league? As soon as Generation VI is announced, which should happen soon, the incentive to promote the Unova games will be heavily decreased. If anything, I see the writers creating original plots even more than they've been doing so far.