Having them promise that they'll go to Alola seems telling to me since they could have just told Ash that they'll see him when he returns to Kanto. It certainly means more than an ambiguous mention of other regions in an interview.
And I don't understand why the Mega Stones are even remotely interesting. They're just a plot device that requires no explanation. The best way to utilize Misty is have her help Lana, who looks up to her.
Oh, I agree with that last bit. Heck, I even thought that, if anyone is to help inspire Misty to regain her Water Pok?mon Master goal, it's Lana.
So far as Mega Stones, to be fair, Misty's put on a pretty tight leash, so having her have a Gyaradosite makes little sense when her sisters probably can't even let her go to Pallet Town for a prolonged period of time based on the beginning of Battle Frontier before calling her back, much less let her go to Kalos to get the stone there.
EDIT: BTW, the Japanese subs for this episode make it very clear that Ash, if anything, is the first challenger to not only break through Mega-Gyarados's attack, but also actually BEAT it, so if anything, that actually makes Misty's battle against Ash, heck, even a lot of her skills during this episode, even MORE impressive than initially thought, since it implies she had a lot of trainers LOSE against her. Well, she most certainly redeemed the Gym Leaders' name after this, potentially rivalling that of Brock's impressive battle against Kiane. After the crappy treatment Gym Leaders got in AG, BW, and to a lesser extent DP, they needed redemption, and they got it.
Here's the sub proving this claim:
https://mistyisbae.tumblr.com/post/165628826670/kimmy4444
On a related note, here's the sub showing the promise, in case anyone is still in doubt:
https://mistyisbae.tumblr.com/post/165629117435/marsy3-its-a-promise
BTW, that's not my tumblr account, but it is someone I follow (my tumblr account is more along the lines of "otnesse").