Well as far as I'm aware of, Talia is older than Nyssa. So, with that being the case, wouldn't that make Talia the real "heir to the demon"? With the way Nyssa has been introducing herself, it seems as though she's Ra's only child.
Plus, what's the point of introducing the idea that Batman exists within this universe if he's pretty much off-limits? It's like a bigger tease than what Smallville did with Tom Welling and the Superman Suit.lol
Actually, Nyssa is older. Nyssa is supposedly his oldest daughter, though I think "oldest known" daughter is more accurate considering Ra's age.
In the comics, Nyssa was born in 18th century Russia. Talia was a modern born kid for the time. Probably his youngest. Talia's probably supposed to be in her 20's, if she exists
It's worth remembering that in "Uprising", we did see Nyssa as a child approach Malcolm where in which he got his name "The Magician". So, Nyssa's probably not anywhere near as old as she was in the comics. I think there was mention that Nyssa had like twelve children, too. Which is something that I wouldn't expect, like her age, to follow the comics on the show.
I think there's a danger in name dropping, creating expectation, and not delivering. The moment Bruce is mentioned, or Batman is mentioned, you just know the headline on sites will be "Is Batman Coming To Starling City?" or Twitter will explode with #BatmanStarlingCity or something. That hype would be great, if they could actually have him show up. But, if they can't...you'll just disappoint people who will get excited by social media and click bait on websites.
My guess on Talia, is that she wouldn't be a prominent character. She's tied to Batman more than Ra's al Ghul is. Ra's has been featured in Green Arrow comics, and been an adversary of the Justice League. Talia, even when DC Comics has mishandled her (*cough*sleepingwithJasonTodd*cough*) has always been seen as Bruce's "beloved". It might just be one step too close to Batman than I think they might want to get.