developing oil reserves, some focuses about reforming Texas rangers, focus on Texas getting all of its originally claimed territory. etc.

I already have it written down that they claimed most of new mexico

Texas rangers I like it

Maybe a foreign policy focus tree that allows Texas to become a fascist state under the "Nationalist Party". Party leader can be a fictitious descendant of Mirabeau B. Lamar, and focuses requiring fascist government that allow claims for Texas cores all the way to the Pacific (New Mexico, Arizona, Nevada, California)

As a native Texan, this is very exciting. Definitely have Texas be able to retake claims up through New Mexico and into Colorado even. I think it'd be neat to have a "Remember the Alamo" focus that gives them a wargoal on Mexico.

Maybe have a fascist tree allowing them to reform (Like the EU in Millennium Dawn) or make a faction for the confederate states.
What Semaj38 said sounds cool too.

You should add a custom focus tree to Texas that allows it to regain its old land from Oklahoma, New Mexico, and Colorado. There should also be a feature where Texas can go to war with Mexico to reclaim territory from them. There could be so many other features added to Texas, these are just the best in my opinion.

I really like Semaj38's and Golden_Comet's ideas

A split in the focus tree where you can be the former Republic of Texas, or you could become a new fascist Texan Empire (maybe a third option to be in between those two ideologies?) The republic could side with the allies or other democratic U.S. states or if they're fascist they can side with the axis or other fascist states. Through both of these trees you could also try to influence the surrounding states and eventually, you can go to war with Mexico with both trees. Or you could befriend them if democratic.

A split that goes: Honor the Confedacy OR New Texan Empire

If Mexico ever shows up, then they'll need a focus to reclaim Texas and their other lost land.

Something about the German settlers influencing Texan foreign policy could also be interesting, as many Germans settled Texas in the mid 1800s

before texas became apart of america it had more territories look them up for when they were independent and it should show you the lands that they use to own

I like what ITurn Gamer suggested with the split in the focus with the Confederacy and New Texan Empire, but maybe for the New Texan Empire (maybe the name should be something along the lines of the Lone Star Empire or something close to that) instead of going East towards the old states from the Confederacy, you can go West toward California and a bit North for the old Texan lands at Texas used to inhabit.

I also think that Texas should start with some bad national spirits like the Dust Bowl or a lingering Great Depression and you have a economic path to take to relieve of said national spirits in order to make things a bit more challenging and somewhat equal as you are next to small and possibly weak states in the beginning like Oklahoma or New Mexico.