I've read through everything that was said here and noticed some poor excuses for Misty not coming back.
1)Max needs to reappear.-Max is only seven, and the first episode of BW revealed Ash is still ten, which means Max is still seven and would need May or one of his parents to reappear
2)The VA excuse-VAs are changed all the time
3)They would need to bring back all 40 gymleaders-I doubt all of them would leave their gyms at the same time for a tournament that is so far away unless they were forced. Also they could some kind of special tournament maybe for water gym-leaders, which would also allow Shizui to make his debut. They make up interesting competitions all the time for example, the wallace cup, don george tournament, and the donamite.
4)Gary and Brock would need to return as well-Gary just appeared in DP multiple times, and BW marks the second time we haven't had Brock as a traveling companion. There's not much that can be done with him since he has been around almost as long as Ash.
5)Volkner would need to return-I've seen this multiple times, I don't get the importance of Volkner. He appeared in three episodes, and acted a lot like his game counterpart. He's no more important than the other gym leaders that were not Ash's traveling companions
6)A reason is needed to bring her back-If they're bringing Dawn, a coordinator back, they don't need a reason.
7)She's not mentioned at all or on the poster-it could be something for later in the season. Dawn set to appear in the first few episodes, plus it hasn't been stated why Dawn is returning. Cynthia is probably going to appear early as well.
8)She hasn't appeared since 2006, she is forgotten-One other person mentioned this: In the episode where they catch buizel Ash is using Misty's lure, which I think it should be noted that he refuses to let Dawn use it.
As a Misty fan I truly hop she makes an appearance, especially since BW presents plenty of opportunities. I would also love to see her interactions with Dent and Iris, and even Dawn if she's around for a while