In my opinion im of belief how Misty and Iris meeting would make for wonderful encounter.
Brock and Cilan met each other and given how BW mirorred OS in some aspects it would be only appropriate that this two girls meet themselves too with things coming full circle in way.
Both have quite a few things in common striving to become masters of specific type (dragon and water). Tend to sometime be sassy and cynical such as tending to tease and be curious by nature liking to enter various events and try out different things which could foil ground for both finding common language and becoming good friends. Than again certain friction emerging between them shouldn't be completely ruled out either for comic purposes with differences between them and Iris bratty behavior possibly pushing some of Misty buttons causing annoyance or conflict from her side(just to spice things up abit). In relaxing, harmless way misunderstanding each other at first.
While exchanging experiences and talks over various memorable moments, awkward situations and trouble they experienced while traveling with Ash. But sharing future plans and willingness in becoming recognizable and successful type masters eventually too.
Battle between them would be welcomed having quite a few strong pokemon and lot of experience.
I can already imagine Dragonite vs Gyarados being one hell of a fight, or perhaps Misty Starmie which could possibly be much stronger nowadays. I can't discard chance of Misty having some new power addition in her team from 4th or 5th generation pokemon as well.
Or Misty meeting Serena.
As ironic as it may seem both have sharing points in feeling something which appears to be more than friendship toward Ash making people intrigued to see hows those two would cope with differences learning to develop close friendship. Come from families which tried to force them to live under imposed restrictions deciding to follow their own path and showed to be intelligent, quick thinkers in main cast being good at coming up with plans and ways how to fix problem. Yet at same time both are like complete opposite of each other in terms of personality, agency and interests usually being recipe for very good dynamic.
Misty headstrong, playful and fiery nature knowing how to stand up for herself and others could help Serenas to become more proactive, develop inzerest for career she wants to follow and be more confident and assertive taking role in action. With Misty experience and attitude feeling sympathy helping Serena out.
This answer and choices might be induced by bias from my side, but than again usually when expressing desire to see something happen in show is influenced by our own personal affinities and interest toward particular character, rival, pokemon etc. So i cannot be blamed for this.:p
