I think that if Arianne marries, it won't be Aegon. Rereading her chapters, I noticed she likes to think herself of Nymeria reborn. But what if she resembles the first Daenerys more than she does Nymeria? There are some shared characteristics between the two: both loved by a bastard named Daemon (Blackfyre, Sand), who asked their guardians (Doran, Daeron) for their hand but were refused. So what if Arianne is actually Daenerys I reborn? In that case, she wouldn't marry for passion or love, but for duty. I don't see Aegon fitting that image.