I guess a big part depends on which direction you want Texas expanding. There's stuff like The Stars At Night TL where Texas and the USA are allies in the M-A War, basically allowing Texas southward expansion on the back of American military prowess without themselves being annexed.
If we for some reason butterfly the M-A War (and delay the gold rush?) then Texas has the possibility (but not guaranteed) of expanding westward.
One idea might be to push back the gold rush until very soon after the ACW* ITTL. Thus "Confederados" leave the USA for gold in California - this creates a Southern population on the west coast that might be friendly to Texan annexation (especially if it means regaining the right to own slaves - which Mexico doesn't offer).
If we for some reason butterfly the M-A War (and delay the gold rush?) then Texas has the possibility (but not guaranteed) of expanding westward.
One idea might be to push back the gold rush until very soon after the ACW* ITTL. Thus "Confederados" leave the USA for gold in California - this creates a Southern population on the west coast that might be friendly to Texan annexation (especially if it means regaining the right to own slaves - which Mexico doesn't offer).