Running for Governor would be a mistake since Abbott is literally the most popular politician in Texas. Senate would still be a great challenge for Castro but out of the two, Senate is more likely to be won by him.
What about becoming mayor of SA again?
According to this story from November he's thinking about running for county judge in 2018 (and then working his way up to statewide office some time in the 2020s) or else just run for president in 2020. Though now he says he's not going to run for anything in 2018...
Castro is going to wait & Run for President in 2020... Which has little downside...
Possible Outcomes:
1) Depending on the political Climate in 4 years, he could get the nomination
2) He gains a lot of traction, wins states like CA-FL-TX-NV-AZ-UT-NM-CO-Puerto Rico, and finishes 2nd in the Primary & is the natural choice for VP.
3) He loses the Primary, but Dems win the President... and Castro gets a higher profile Cabinet Position.
****The only real downside.... Would be if Trump gets re-elected