If so, it would make the most sense to rebuild there. I bet by then, they'd probably find a new home for the Paleontology museum too. Maybe even build a new museum somewhere to house both the Bean museum and Paleontology museum as one bigger life sciences museum. They'd probably have to re-home the UPC next to the Paleontology museum as well.
However, when you look at Google maps and measure the distances, width of the current stadium from east to west exceeds the east to west width of the parking lot, so that might not actually be a viable option anyway.
The next best option that would be more viable would be the old Provo High campus. However, I think that is destined to become the new med school, given it's proximity to the hospital, among other things. Parking and traffic flow would also get far worse with that location.
But beyond those two sites, I don't see any other viable lots large enough to accommodate a new stadium without significantly major relocations of other campus buildings and departments. Using that lot that the old Provo Temple sat on could have been an interesting location to move the stadium too, and would have provided an even more intimate view of the mountains, but that's obviously out of the question now with the rebuilding of the new temple.
So yeah...if they ended up just using the existing lot, they'll have to undertake something similar to the scale of what they're doing with the Salt Lake Temple, and that would be so much more expensive. It would also be more disruptive in that it would probably require BYU to temporarily relocate its home games for at least a season or two. And I don't know if anywhere that you could do that other than RES, which, ummm, no, not happening and the scheduling would get more complicated with that too.
If they were able to do a reconstruction in phases, section by section, like how they're managing to do at other places, like Vanderbilt and so forth, that will also be really disruptive to season ticket holders and all that.
Expenses aside, there's still no easy and convenient way for a new stadium to be built no matter how you slice it. The logistics will be a massive undertaking.