Volkner's appearence would make sense and not. If pointing that Jasmine went to Sinnoh and then her Ampharos got sick, the storyline in G/S/C would be after Jasmines return from Sinnoh, it would be better if Jasmine would go to Sinnoh when Ampharos is OK already. Then she would came and mention Sinnoh and even Sunyshore.
I will also remember that Elekid is only obtainable on Crystal when the daycare old lady gives you an egg (with 15% prob. of being shiny). Other than it would be breedable from an Electrabuzz caught on Kanto (If there will be Kanto + After Elite 4 so you couldn't have an elekid on Johto).So,
Volkner could be visiting Olivine (not Cianwood --.), maybe even visiting Jasmine (poiting that they're both Gym Leaders so it is possible that they are aquittances/friends), and the reason would be giving the player a Pokémon Egg, Elekid's Egg (why not?).
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About Flint's cameo: Why Flint? Why not Flannery? There is no reason for Flint to make a cameo, I'm not saying that it is impossible and that it makes no sense, but in case it has no sense.
Last, I shall remember everyone that ANY character may make a cameo on this Remakes, or did you even wonder that Jasmine would appear on D/P/Pt?