Currently, I don’t endgame Hapu with anyone nor does she have any children. I don’t think that will change, either, since I see her as someone who is happy to remain single. 

While she has no children of her own, she has a relationship with a few of the Alola fankids. In particular, she’s an aunt-like figure to Lehua and a mentor figure to Ohia, who eventually succeeds her as kahuna of Poni Island (though that’s a long ways away.) 

Additionally, she plays a role in Echter’s storyline (aka the main Alola storyline), though this is less fleshed out right now.