Originally posted by Rei_Zero
Also, Haruka probably started liking Ash much quicker than Misty. She nearly popped into his room on the first eppie. Misty didn't show any obvious signs until like 20 eppies later.
Well I personally think her talking to herself about Ash's virtues in the Metapod vs. Metapod episode was one of the most obvious signs that she stopped following Ash because of the bike long ago. Of course, that still means Haruka became fond of him even faster.

This might not be Misty's last special until next Christmas. Originally, they only did specials every year at Christmas time. Now, with Hoso, they might have one more often. So, maybe Misty will more or less "get her own series."
I somewhat doubt this, but regardless I think it extremely unlikely that she will be leaving the show permanently. I'm almost certain the writers simply want to have Ash have to do without Misty's guidance, as well as set up for an inevitable confrontation between Misty and Haruka.

Brock's departure had a set up where he could have left for good but Misty certainly doesn't. I suspect her sister's world tour would last, at most, a year, after which the egotistical trio will likely take over the gym again. Where the show leads then is anyone's guess.