I was thinkin San Andreas style with three cities and three rural areas. Desert Mexican South Texas/San Antonio, Forest Black/Cajun East Texas/Austin-Houston-Galveston hybrid, Plains White/Injun North Texas/Dallas. Lot of stories to tell with cartels, but a ton of human variety to play with.
Edit: Think about it, name a game set in Texas. You would think there would be a shit ton, and there are none.
Realistically the best way of doing Texas would be South Texas + Tamaulipas and Nuevo Leon with San Antonio and Monterrey serving as the main axis.
You could use the Guadalupe or Colorado river as the northern boundary then using Hill Country to hem in the North and west boundaries of the map north of the Rio Grande then the Sierra Madre Oriental for the western and southern boundary on the Mexican side, similar to how the Mountains are used to hem in Blaine County
Also to note Tamaulipas outside of the Rio Grande Valley is pretty much empty much like Blaine County is between Route 68 and Los Santos.
Meanwhile It'll even let you have SpaceX and South Padre Island too, and if they want to be a little bit more ambitious with Hill Country they can try to include Austin to connect SpaceX and the Tesla GigaFactory but I'm not sure how relavent Elon Musk will be in 10 years as it seems he's waning.
You could name Texas "Bexar" which due to having the same amount of letters and vowels and X in the middle it is almost perfect so Texan becomes Bexan, Bex-Mex, BxDOT and BxDPS etc
Also it'll piss off everyone that lives in San Antonio since Bexar County is pronouced "Bear" because it's a Spanish name but non-natives pronounce it Beks-sar like Texas which reflects how Texas went from being Teyhas in Spanish to it's current English pronunciation.
Tamaulipas I could see becoming Tamalepaís which is broken Spanish for "Tamale Country", or if they don't want to be that lazy, Nuevo Santander the colonial name for the area
Nuevo Leon would of course become Nueva Castilla
All in All besides the main cities, you'll have Laredo, Victoria, Corpus Christi, South Padre, New Braunfels, Eagle Pass as well as the Rio Grande Valley to play with.