I agree. They've basically done this kind of dynamic before which was somewhat successful like Sophie x Plachta (although I didn't see them in a romantic way since they were more like apprentice x master). Also for me, Stia's character was rather... bland and uninteresting.
Actually, there are a couple other Atelier games that had repeated protagonists such as Atelier Iris and Khemia. But to the main topic, YAS KLAUDIA X RYZA WHOOOO! Make it happen GUST! Klaudia seems to have a palpable role in the sequel LET'S GO!
Totori is great! The characters and story are better than Rorona and Meruru in my opinion. And the yuri subtext heh. Though Meruru's yuri subtext is even greater.