Either two are not needed.
Neither is Cheren but he's getting a scene. And late too.
Cheren ends up becoming a gym leader so it makes sense showing him since all other gym leaders (bar Iris) have been shown, but both Rosa/Nate or Hilda/Hilbert are just trainers, so not much can be done with them.
And yet in sinnoh, Lyra had a role. There are many possibilities, you just have to think outside the box.
Lyra's role wasn't much of thinking outside the box, she briefly appeared and then was gone again without leaving much impact. In a series DP's size I'm not complaining about that, but for Best Wishes (especially a season that should focus on TP) throwing Hilbert/Hilda or Rosa/Nate in the series briefly just for the sake of it will spite more people than it will appease.
Actually we got lyra so the writers wouldn't have to make a johto filler arc like BF
Only Hugh has a chance to appear in my eyes.
I mean, making some connections, Cilan has some past with Purrloin. Hugh is looking for that specific Pokemon... Maybe, just maybe, Cilan will receive character development through Hugh.
She appeared to promote HG/SS.
I don't know, usually they use the protagonists in movie cameos, and we've failed to see them. But I could see Rosa and her Serperior fight Cheren in the beginning of the episode when Ash and Co. appears.
We already saw Serperior many times. I don't think we need to see anymore.
And since the Team Plasma arc is starting, Hugh might be the most likely to appear, if you ask me (though his storyline in the games really annoyed me)
I wonder if he'll unleash his rage.