I think a good Davis map would be the usual Democratic strongholds (Mexico border, San Antonio, Austin, Urban Dallas) plus a stronger margin in Houston County than other Democrats have gotten. Plus I'd expect her to do better in Tarrant Co. and the Corpus Cristi area.
Tarrant County is a good bellweather for the state as a whole, so I'd expect her to need to do well there in order to have a ghost of a chance.
I assume that by
Houston County you mean
Harris County, since Houston County is a small East Texas county with relatively few people and generally votes safely Republican. Harris County is generally a 50/50 county, so it's not going to do much more for her than cancel out the votes that Abbott gets there.
I expect her to do better in Tarrant County than previous statewide candidates. Bill White got 41% of the vote there in 2010; Barack Obama got 41% of the vote there in 2012. She'll need to do two things: (1) Make herself acceptable to white suburban women; or, making Greg Abbott seem completely unacceptable to them depending on how he behaves during the campaign. (2) Get Hispanics to vote...period. Regarding the first, she ought to borrow heavily from the playbook of Laura Miller, the liberal newspaper columnist who was elected mayor of Dallas in the early 2000s with a coalition that depended unusually heavily on moderate Republican women.
The Coastal Bend area has been trending Republican over the past few cycles, so Corpus Christi is not a given for the Democrats.