Although, I think McCaul might be a choice because there's a chance for his district to get severely more competitive, more here.
So a DHS position might be a helicopter rescue out of an impossible district.
I'm not sure exactly how fast that office will be filled, as we all know about Trump's never-ending vacancies issues, but if Texas is to get a new Congressional map, it won't be happening for what I imagine will be months. I'm not even sure the trial over the 2013 interim maps has concluded yet, and there still has to be a redraw process which will take time.
That being said, DHS doesn't seem like an office Trump would leave empty for that long. So all I can say is, I sure
hope he picks McCaul. At least right now, that district doesn't seem so R-leaning that it is guaranteed safe in a special election, at least given what we have seen so far.