If the Summit loses everyone east of the Mississippi River, could it pull off the following?
South Dakota State
North Dakota State
North Dakota (from Big Sky)
Montana (from Big Sky)
Montana State (from Big Sky)
The first six establish the Summit as an FCS conference, Denver is currently the seventh Division I member, and Omaha will make eight once their transition is complete.
The Big Sky is left with the following;
Idaho (FBS football)
Cal Poly (football only)
UC Davis (football only)
The Big Sky could take Seattle if it wanted 10 members in basketball, or simply have Idaho not join. Northern Colorado could head to the Summit as well.
When I first joined this site one of my first posts more than two years ago was the need for a second western FCS conference. Fullerton seemed to be thinking the same thing last year and there was open speculation of a WAC-Big Sky alliance, however it failed pretty quickly.
However, that did seem to suggest that the Big Sky schools want the other three Dakota schools and maybe more, but they don't want to make it even harder to get an AQ, especially in olympic sports. The Summit losing a bunch of schools could give Fullerton a second chance at creating an ally conference, and it could look very similar to what you describe. I would put N Colorado in the Summit personally, but that's not a big issue.
Now, the Dakota schools moving will have nothing to do with their football home which is solid. Any move will be to stabilize their olympic sports. This is why ultimately I believe that the MVC will need to offer full invites to at least NDSU and SDSU, if not USD as well.
So I'd say the chances are slim, but I bet something like it would be considered if the Summit loses a lot of their eastern schools.