They’ve already announced a huge casino/arena project south of the strip. Pretty sure FSG isn’t involved but that’s supposed to be where the NBA arena is being built. I think it’s the LA-based Oak View Group who is building it.
You seriously underestimate just how much of the Las Vegas population has come from Los Angeles and the rest of Raiders fandom. Plus the cities are close enough that it’s easy for LA residents to make day trips for Raiders games. There are tons who already do make weekend trips here since it’s just a 4-5 drive (or a quick cheap 1 hour flight).
That is a different project, there is a couple different groups trying to bring the NBA to LV