I've gotten a couple of suggestions that Cate and possibly Kitty and Chalise all need to get pregnant, since they have a more personal connection to Alex, and if anyone deserves to bare his heir, it is one of them.
The only problem with that (aside from the fact that he hasn't seen Kitty and Chalise for a while), is that being young idealistic young women, they're all on birth control until they finish college (at least). Thus it would take an accident for that to happen. Stranger things have happened, so it's not an unlikely thing to pull off, but none of them are exactly looking to be baring his babies just yet. (They always have access to his sperm if something happens to him before they're ready).
What's more, this is simply a numbers game at this point. The idea is, they don't know what it'll take to hit the right genetic combination, so they're trying as many options as they can without knowing how successful any single attempt will be. They'd Love a combination Alex/Seer trait, although they're aiming simply for another Alex type from anyone. Basically, the future of them as a people rests upon that happening, so none of them (Cate, Kitty and Chalise included) are overly concerned that it be one of them.
It's further complicated because no one knows what to look for. They only learned about Alex once he went to New Orleans at the age of 17. Cate reports she could feel his energy since she was young, but no one knows when that might have been, or whether it was simple wish fulfillment or not. Thus no one really knows what to look for. Clearly, seeing a baby glowing with a bright aura would be idea (aka the dream from chapter 40), but no one knows whether that is likely or not. After all, did Alex's aura ever glow that bright before he first establish his link to another dimension with a higher concentration of BCM?