Leliana definitely seems to have a soft spot for children, hence Kieran and she also has that one conversation that is easy to miss at the Winter Palace with a noble friend of hers who tells Lels his children will be overjoyed to see her again and Leliana expresses excitement about visiting them as well.
There's no doubt she'd make a great mother who would strive to be there for her kid(s) because she lost her parents at a very young age and wouldn't allow them to relive through similar things.
Though if the Warden or anybody else asked her whether she'd like to have children sometime in the future I think she'd have second thoughts about it. We know Leliana is very self-critical about herself, sometimes too much for her own good, she'd say she would be terrible as a mother since she never knew her own and wouldn't know how to properly raise her child. And given her life-style she probably gave up the hope wanting/ever having a child because of the dangerous life she leads.
Except Kieran, I still headcanon my Warden having at least one child with Leliana after returning from his Quest.
If Gaider said that Alistair/Anora and Alistair/Warden will most likely produce an heir one day, I don't see why any Origins LI wouldn't be able to have children. The writers wouldn't take that away from us.