To be honest if we could play teams tougher, then anyone who currently doesn't have one. Just because SMU doesn't look to us as a "rival" doesn't mean they wouldn't after we beat them a few more times. Same for Houston and Rice. I like to think that we would set our eyes a little higher than FBS schools to have anything to do with.
Come to think of it, I don't think we need an intra state rival. What other texas schools have one? The only thing I can think of is TU and AtM and even then TU doesn't consider them much of a rivalry anymore. I think the intra state rivalries are mostly in smaller states with usually 2 major universities. Auburn Alabama, Ole Miss MissSt, Clemson SC, Iowa Iowa St, Kansas KSt, etc. For texas Baylor doesn't look to TCU or Tech as a rivalry, Tech wants to rival with AtM, SMU wants TCU as a rivalry.
For a "rivalry" to occur we need to consistently schedule an opposing team almost every year. If we had SMU/Houston/Rice scheduled every year a rivalry would be born.