Yeah it could have been suggested, but ND would never agree to it. There's kind of a huge underestimation of how important football independence is to ND. The bandwagoners might not place much emphasis on it, but I know one fairly large ND booster and am friends with some alumni and they would have all scrapped playing any games in 2020 before joining any conference for football. It might not make sense to us, but it's a huge deal to them.
The only way ND ever joins any conference in football is if being in a conference is the only path to winning a championship.