If Texas were ever crzy enough to dismember itself, here is what I would recommend.
1) State of Galveston (capital Houston)
In the extreme SE corner, with the Brazos as the western border. and then proceeding from the intersection of 31N and the Brazos to that of 94W and the Sabine on the LA border
2) State of West Texas (capital Lubbock)
The panhandle, consisting of that part of Texas west of 100W and north of 32N.
3) State of Pecos (capital Pecos)
All of Texas west of the Pecos, plus that part of Texas between 30N and 32 N that is west of 100W.
The remaineder of Texas would split along 31 N between
4) State of North Texas (capital Ft. Worth)
and
5) State of South Texas (capital San Antonio)
We did a similar exercise as part of a repartition of all 50 states awhile ago on the forum. In TX, I had the following splits by county:
East Texas (2000 pop 6.7 M): Brazoria, Fort Bend, Galveston, Harris, Chambers, Liberty, Montgomery, Grimes, Waller, Jefferson Orange, Hardin, Newton, Jasper, Tyler, Polk, Sabine, San Augustine, Shelby, Angelina, Nacogdoches, Rusk, Panola, Gregg, Harrison, Marion, Cass, Bowie, Red River, Morris, Titus, Franklin, Camp, Upshur, Brazos, Robertson, Madison, Leon, Walker, San Jacinto, Trinity, Houston, Anderson, Cherokee, Henderson, Smith, Van Zandt, Rains, Wood, Hopkins, Delta, Lamar.
Central Texas (2000 pop 7.8 M): Kramer, Bastrop, Hays, Travis, Lee Burleson, Williamson, Milam, Bell, Falls, Limestone, Freestone, Coryell, McLennan, Hill, Navarro, Ellis, Kendall, Gillespie, Blanco, Mason, Llano, Burnett, McCulloch, San Saba, Lampasas, Brons, Mills, Comanche, Eastland, Erath, Hamilton, Hood, Bosque, Somervell, Johnson, fannin, Hunt, Kaufman, Grayson, Collin, Rockwall, Dallas, Tarrant, Denton, Cooke, Montague, Wise, Parker, Clay, Jack, Palo Pinto, Wichita, Archer, Young, Stephens.
Lower Rio Grande (2000 pop 3.6 M): Zapata, Starr, Hidalgo, Cameraon, Willacy, Jim Hogg, Brooks, Kenedy, Duval, Jim Wells, Kleberg, Nueces, McMullen, Live Oak, Kendall, Bexar, Atacosa, Comal, Guadelupe, Wilson, San Patricio, Gonzales, Karnes, DeWitt, Goliad, Bee, Refugio, Calhoun Victoria, Jackson, Matagorda, Wharton, Colorado, Lavaca, Fayette, Austin, Washington.
Upper Rio Grande (2000 pop 1.1 M): Val Verde, Edwards, Real, Kinney, Uvalde, Maverick, Zavala, Dimmit, Webb, LaSalle, Frio, Medina, Bandera, Kerr. In the earlier exercise this was combined with most of NM and south central CO (Pueblo area) for a state of 2.8 M.
West Texas (2000 pop 1.6 M): everything else in TX. In the earlier exercise I compined this with eastern NM and most of OK (except the eastern 1/3)for a state with 3.8 M.