Assuming the MVC even wants to expand.
Looking for schools in the greater MVC footprint:
Butler (Indianapolis) Horizon League, Pioneer FB
Oral Roberts (Tulsa) Mid-Con, no FB
U Missouri Kansas City Mid-Con, no FB
Western Illinois, (Macomb) Mid-Con, Gateway FB
Valparaiso (Indiana) Mid-Con, Pioneer FB
SEMo (Cape Girardeau) Ohio Valley, plays FB
Eastern Illinois (Charleston) Ohio Valley, plays FB
Eastern Kentucky (Richmond) Ohio Valley, plays FB
Murray State (Murray, Kentucky) Ohio Valley, plays FB
Western Kentucy (Bowling Green ?), SunBelt, Gateway FB
and finally the NDSU / SDSU (pair).
Assuming SLU considers themself too "big-time" to join the MVC, I would think any expansion candidates would come from the above list.
If the MVC is going to 12, I would think goals might be to consolidate the Gateway into the MVC (reel in some more I-AA FB teams) and minimize travel costs.
The former would suggest WKU, WIU, or an Ohio Valley School. Not sure which schools out of that sub-group would be most attractive.
Other than Western Kentucky none of those teams MVC wants. Go to valleytalk.net and they have a discussion on this. Even Western Kentucky most fans dont want.