I wouldn't be opposed to Dawn and Serena meeting. Im sure they could click of well establishing versatile, fun and dynamic relationship. Maybe Dawn could help Serena to choose better dresses for pokemon performance, pass on her some of knowledge she gathered by meeting pokemon stylists like Paris and her Lopunny.
Now that i recall Dawn herself showed interest in designing clothes and accesories. Comparatively pokemon performance could arguably help Dawn to pick up some unique styles and glamurous presentation of pokemon and trainers on stage which could be of help for her future contests. By using acquied knowledge in putting more elegant execution of her teamwork with pokemon during appeal rounds.
BUT more i think about it, the more i see exact same scenario happening like it used to be csase with May and Dawn meeting at Wallace Cup. Showing two girl girls with similar personality traits, similar interests in adoring fashion, cute dresses and making performance on stage with their pokemon(through contests or performing).
Not delivering anything groundbreaking and unpredictable as far as plot progression and character interactions go.
If anything i would be far more interested in Misty meeting Serena. They are very different from each other which would provide challenge in building on good friendship. Since both girls perceive world and priorities highly different from one to another. With Misty being more adventurous, rebellious and strongwilled type of girl. Meeting up with Serena who is in many ways exact opposite being introvert, far more girly and shy type of person.
There is also potential that such meeting influence storyline to notable level, due to Serena and Misty being recognoized as 2 main girls which developed or had/have love feelings for Ash.
Foreboding possible friction or tension in relationship at first with both girls coming to misunderstanding themselves and potentially jealous. Having to learn how to overcome differences, find coimmon things and actually working for creating strong friendship. Rather thsan haoppening out of blue.
With two girls meeting and becoming BFF's right away.
Misty can help Serena in emotional sense more asw well. To build stronger confidence reinforcing belief of poke performer being right path she choosed and stand up for herself being good in motivating others, even more so after Serena see her in action in taking care of some crisis or battling.
Now im not saying how with Misty it could be any different, but based on her common thing with Serena in regards to Ash question and more ardent personality, Chances for different, more unique type of interactions going between older and newer companion meeting each other are much higher.
I mean i like Dawn and everything, But she appeared for 9 episodes in BW. While Misty got what again after 10 years? Answer is nothing, zero, nichts ever since her last return in 2005.
Having unfinished story, unresolved dreams of becoming water master and reach E4 level along wth uncleared situaton about gym and her future, Has more enjoyable, full of unique emotions personality and Kalos offers more material to develop her more(than it could do for Dawn).
Like learning how to mega evolve her pokemon like Gyarados(being important for type masters to know to use pokemon to full potential). Meet E4 Siebold who is first elzte water trainer in game history classified as water master. Enter strong tournaments and making something oub of story coming closer toward her dreams achievement.
So while i wouldn't be against Dawn meeting Serena, im fine with it. There are more interesting and imo better options when it comes to past female companion meeting Serena.