Could be either. Though, realistically I'm leaning more towards the fact that Serena and Ash met a couple of times and she was inspired by him to become a trainer, moreso than childhood friends. I mean she doesn't refer to him as "Satoshi" in the trailer, rather she calls him "that boy from back then", as transalted by Adamant.the writers could really troll, and have the boy she met in the past not be Ash..rather just someone that looked like him IDK. As for Ash and Clemont...Ash will break Clemonts gadgets and Clemont would get pissed, and Bonnie would ship them bring them back together.
don't tell me you already imagined in your head Ash and Tieruno dancing togther -_-
tears of joy? b-but i thought you wanted Ash x Iris.
Of course we are. It just seems like a nice setup. Serena likes Ash. Bonnie is apparently looking for a suitable wife for Clemont, and Ash obviously won't return Serena's feelings. So Serena/Clemont seems like a viable direction for the writers to take. Your idea is pretty good too tho. XP
that or when Iris cameos it'll be Iris vs Serena over Ash, while he just sits back wondering wtf is going on