After all originally pokemon was intended to end after Kanto, but since games, show and whole concept became enormously popular producers decided to make sequel leaving this footage and material for ending unused.
Many people say how that little girl with pink hair was considered to be "Ash x Misty daughter" with presence of Pikachu adding more fuel to such theory. Especially given all those hints of romantic affection Misty showed toward Ash in OS.
However since we know ex head writer was against idea of romance in show makes such theory highly questionable and ungrounded, but than again Shudo himself mentioned how not everyone shared same views as he did resulting in disagreements and anime not taken through direction he wanted.
Personally im rather indifferent to this anyway because im not shipper, but i can see why so many people speculated about this given show history and what was present in trailer.
p.s. What i find more interesting however is that this was probably closest as we are going to get to see main characters aging, with time skip being used for this trailer.
It also seems Misty was only main character from pokemon we got chance to see as older, being one of possible ways how she could turn out to be in future.
