Building another university is very unlikely in my book. The expense would be astronomical and it take a very long time have an ounce of credibility in the academic world.

1 - Small expansion in Provo. With recent land grabs and larger buildings on campus I can see BYU increasing overall enrollment by 10K over the next 10 years.
2 - BYU-I continues to expand. In the end, it might become the largest university in the system. Right now they are sitting at 20K on campus during a single track. I bet their goal is to get to 30K. If they start shifting campus south towards where they have large amounts of land, they could expand the university quite a bit.
3 - Kill off LDS Business college and replace it with an enhanced BYU SLC Center. Expand the BYU SLC center as much as possible. It sits in a easy transit location, so commuting from across the area is easy. Use it to try and take a few thousand students away from Provo or for offering a great Spring/Summer schedule to get kids through school faster.
4 - Most drastic and unlikely - instead of starting another school, create a satellite BYU campus in a place like Mesa. This way you don't have to accredidate from scratch and there is sufficient local population to attend, support, and staff the school. UoA and ASU have been very successful doing similar in downtown Phoenix. Start small like the SLC Center and see where it goes.
5 - Get BYU and BYU-I to cooperate better. Align majors/programs/general eds where possible to make transferring a bit easier.