Has it been ruled out that Houston could expand its elephant facility?
I'm sure the zoo doesn't want to be too limited to Asian elephants, which are expensive, but it does seem like they could add space where the Okapis, Bongos, Blue Duiker, Tortoise, and Zebra (which used to be with the giraffes once already) currently are. Obviously not the whole space, but they could easily expand it that direction more, if they wanted to increase their Asian Elephant capacity.
They could also shift the reptile house somewhere and expand that way, if they wanted to up their capacity. It does seem like reptiles and snakes are being found in other areas of the zoo now, with more recent additions.
It's definitely something that the zoo needs to be thinking about if they're not looking to do any female transfers. Which as already has been said, a transfer will be much more difficult if Shanti and Joy have calves, do they move "the T-line" in that case? Splitting an established herd like this is difficult - without breeding going on.