Will it be co-signed I-69/US-59? If so, will the US59 eventually be dropped?
Good question. *So far*, all the parts of US 59 with new I-69 (as well as I-369 in Texarkana) signage/shields still include US 59 signage/shields for now. Nothing has been said officially yet as to the future status of US 59 along the corridors where I-69 and I-369 (plus I-69W and maybe I-69C later on) will overlap the US route. It could very well be that eventually, US 59 could be mostly or completely phased out in TX. If that were to happen, who knows where US 59's southern terminus would end up (Texarkana (but with the way US 59 gets treated in AR, maybe not)? Or somewhere in OK?).... For now though, I don't see any US 66-scale (or US 81 Fort Worth-Laredo, or US 75 Dallas-Galveston, etc...) uprooting of US 59 anytime soon.
((The same conversation could be true about the 3 US routes (77, 83, 281) involved in the Valley's interstate additions, but again, so far, nothing has been said about any future removal/shortening of any of the 3 US designations for now.))