BW Cameo

I don't think it's completely busted because of Serena did get a special, she'd never return and just get a cameo.

But we know that her voice actress is working in the series since as early as August or September (which is confirmed). Whether she voices a Pokémon or a human character, the fact remains that she voiced nobody else BUT Serena since her debut as Serena. Just like how Dawn's voice actress used to voice characters of the day until her major role as Dawn. She can appear as early as late November or anywhere around December, maybe as late as January. We know this because Bonnie's voice actress confirmed on Twitter or a blog that episodes are aired 3-4 months after they finished voice acting. And Dawn's voice actress indicated on her 2012 interview that she'll definitely appear and that she wasn't leaving permanently. So while they won't get their specials, it's likely that they'll be reintroduced in Alola.

But there's another thing, in the SM demo, a man celebrates his Pikachu's birthday on Oct 30th, a girl is leaving Alola on Nov 10th on a ferry, which can be a Dawn reference, there was another NPC who is an obvious reference to Serena, but not 100% the same.

She's not casted as Lana, Lillie or Mallow, so that's a better chance she's going to appear as Dawn. Considering that she hinted at becoming a main cast member again and that she had no other roles in the series other than Dawn after Dawn's debut.

I mean, they had a Dawn reference in the XY series each year (for 2014 and 2015), and she was the only former companion who is referenced in some form in XY, She had an appearance after-credits (mostly clips of a DP episode) and a mention of her Piplup. So it's possible that Dawn (and Piplup) are likely to be reintroduced in 2016, but maybe this month or next month. But while I doubt Serena or Dawn would be in the XY episodes themselves, it's likely they'll just be in Alola right away and also likely shown in SM previews before the debut or during the start of the debut. Why did they mention or reference Dawn in there if they want people to forget about her, even when she didn't get an appearance in an XY episode?

yes!