I would say the SU's will go to the Gateway if asked, and I'd say it's highly probable they will be asked to join. As to the Great West, I guess UND/USD will again make it a 5 team conference and that's where it will stay in the short run.
UND doesn't have to worry about an auto bid for the playoffs, since they will be ineligable anyway for 5 years. By then, the bigger question will be, who is left in the Great West? The California schools could find something out on the coast, and that could be the end of the Great West.
I can't imagine the SU's wanting to stay in the Great West even if they do expand. the Gateway is just too tempting. It's where they do their recruiting, and it's closer then going to California, plus they have the auto bid.
The biggest mistake made was by UND/USD in not pushing to move the entire NCC up to D1 when the SU's wanted to do it. Had they done it then, all schools would be eligible for an auto bid in a year and a half. I have a feeling UND will pay dearly for that decision.