As disgusting as Ted Cruz is, I don't see the GOP turning away from him and the GOP also did well in the mid-terms even though the events over the past two years should have at least made for some tight races which didn't happen.
Julian Castro is somewhat on the outs with the state Democratic party because they expected either Joaquin or Julian Castro to run for governor in 2018 and both declined. If either had been on the ticket it is possible that would have provided the boost that Beto needed to defeat Ted Cruz for senator. Julian's position on open borders with Mexico and prior service in the Obama administration won't fly with most Texans so he would almost definitely be defeated. Also, neither of the Castro brothers have much base support outside of the San Antonio area.
I actually had to look up the other person you mentioned, Scott Kelly, since I thought of his brother, Sen. Mark Kelly instead. While he has a great career, his lack of previous involvement in politics, lack of name recognition, and very weak ties to Texas overall eliminate him from serious consideration as a senatorial nominee.
As much as I would like for things to be different in Texas, currently there are no Democrats here that could be elected to a state-wide office--even the seat held by Ted Cruz.
ETA: When I looked up Scott Kelly, it said that his home is in Colorado.
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