This is something :iconwilldynamo55: showed me - http://www.aminoapps.com/page/pokemon/3484041/reasons-serena-will-return-to-pokemon-sun-and-moon
From what they say, I gather-
1. I see that Hoenn is close to Alola.
2. The white flowers (forget me nots) at the end of XY episode 140 are said to be an Easter egg.
3. Serena DID say she'd return. I'd be surprised if that is the case.
4. Ash noticed a similar ribbon that Serena has while visiting Lillie's mansion.
5. Anime panels from upcoming episodes
6. They might've listened to this petition - https://www.change.org/p/pokemon-bring-serena-to-the-alola-region-with-ash?recruiter=48183303&utm_source=share_petition&utm_medium=facebook&utm_campaign=autopublish&utm_term=des-lg-no_src-reason_msg&fb_refefault
7. They might bring Pokémon Showcases to Alola, who knows?
Feel free to share your thoughts.