Well, they could pull an unofficial crossover, like they sometimes do in the comics, and have some one (or more) of our heroes jumping from universe to universe and ending up in the wrong one, looking in the faces of cast members of Agents of SHIELD and/or the Marvel Netflix shows, without mentioning names or showing any costumes or insignias. Then our hero says something to the effects of "Whoops, wrong universe", and jumps to the next Earth.If we're talking about Marvel crossovers now, I've thought it'd be fun if they got Brianna Venskus back -- "Agent Vasquez, is that you?" "No, name's Piper, Agent of SHIELD."
By the way, Marc Guggenheim recently announced that he'll write the Arrow-episode of CoIE with none other than Marv Wolfman himself.