Cuban would be a really weak nominee (I can't see modern Democrats getting behind a rich guy that easily), but I am surprised no one has been floating his name for Texas Senate/Governor in 2018. He fits Texas Dems and Indies far better than the national equivalents.
If Democrats want to truly expand the map, they need to find someone who can win statewide election in Texas. Right now I think Cuban would be the best bet possible.
Cuban has better things to do than be one of 100 senators or be the governor of one of 50 states.