New Mexico State - YES
I think New Mexico State would be a good addition to the Valley; aren't they a former member, as well? Not sure they bring in much of a market but they are a historically good team and much more of a "name" than the other suggestions. As has been suggested, they can go independent in football and park their other sports in the Valley.
North Dakota State - NO
As a rule, the Valley should not consider admitting teams that are Division I infants (and yes I realize that MSU, UNI, and UE had only been D1 for about ten years when they joined the Valley, but those schools transitioned from D2 to D1 long before it became common to do so), because frankly it just looks bad. Furthermore, the Dakota market is pitiful and the Dakota teams bring nothing to the table.
Oral Roberts - NO
I would caution those tempted to bring another Omaha-sized market to the Valley. This is because ORU would bring the Tulsa market to the same extent that Chicago State would bring us the Chicago market, or SIU-Edwardsville would bring us the St. Louis market. ORU typically fields a decent basketball team, but that school's image and financial condition is a massive trainwreck just waiting to happen.
University of Denver - YES
Denver would be a good addition because they have generally good facilities and resources, and they have the potential to be a solid program year-in and year-out. They are also in a great media market. So I'd say, if we must expand, go to 12 teams and add New Mexico State and Denver.