Whatever this was supposed to mean what im sure is that Pikachu in trailer was without doubt Ash's. They surely wouldn't left out main protagonist best partner from series finale, but that doesn't prove how there was something between Ash and Misty either. Regardless of Misty showing romantic affection toward Ash in OS, it was clearly obvious from start how main writer was never supportive of idea of romance becoming main component of pokemon series.
Granted some could use mr. Shudo disagreements with other members of pokemon staff as implication how not all writers were necessarily against such idea, but even if that was true that wouldn't add much difference since they weren't in charge.
Admittedly theory of Ash dying at end of first movie remaining in stone form does make somewhat sense since tragic incident could add long lasting emotional impact to series conclusion, and it would also explain out of main characters only Misty and Pikachu presence in that trailer. But still that doesn't explain absence of Brock, with some things about this trailer to this day being left unanswered remaining mystery.
Only reason why we don't see them very often here is because many older posters left this places with most realizing how complaining wont change anything, regardless how they may feel about this. Although among majority of online fandom this still seems like touchy subject.
Though regarding this trailer most fans still to this day speculate how that little girl could be Ash and Misty daughter and how series producers could have originally planned to have Ash pair up with someone at very end. Personally i find that highly unlikely for aforementioned reasons, but i have to admit hints from Kanto along with Pikachu and Misty presence in older form with unknown girl can start to raise up various questions.