So, while watching the episodes featuring Virgil, I had a thought. Well, more like a theory.
Virgil already has seven Eeveelutions as well as an unevolved Eevee. And recently, we were shown a new eighth Eeveelution, Sylveon. Could the writers be planning to have Virgil's Eevee evolve into a Sylveon later?
I think this would be SO awesome. I would write it like this: in a near future episode where Virgil reappears, his Eevee finally evolves because of some strange event that's never been known to evolve an Eevee before, and everybody eagerly awaits to see the result. It evolves into this cutesy Pokemon that Iris says looks absolutely adorable. But none of them knows what it is. It's pink and white with bows on its body and has big blue eyes. Ash tries to pull it up on his Pokedex, but it only says the species is unknown. So throughout the episode, the gang tries to figure out what this new Eeveelution is. But unfortunately, they don't have any luck at all. Virgil gets worried, because he can't use a Pokemon if he doesn't know a single thing about it other than that it exists. His new Pokemon is also sad, but knows it isn't Virgil's fault. The Pokemon disappears and tries on its own to find help so its trainer can get to know it again. But it gets in trouble with Team Rocket and Team Plasma (yes, both! The more action, the better!) and the gang has to rescue it. Team Rocket and Team Plasma both see that it's a rare and apparently newly discovered Pokemon and wish to examine it in case it would be useful to them.
However, while TR and the Plasma grunts fight over the new Pokemon at first, they eventually decide to call a temporary truce so they can get Ash and co. out of the way. The combined strength of TR and TP proves difficult for the heroes. Thankfully, a new character (that would be from X and Y's region) shows up and helps even the odds (the heroes were outnumbered pretty good due to the number of TP grunts present). After TR and TP are defeated, Virgil asks the new trainer if they know the identity of his mysterious new Pokemon. The trainer does know and reveals it to be Sylveon, an evolution of Eevee that was recently discovered in the region that he/she's from. The trainer explains all about Sylveon and mentions the name of the region it was discovered in (and a second new Eeveelution if one is revealed). Virgil thanks the trainer and both he and Sylveon are happy again. And we get a traditional episode ending of Ash and co. saying their farewell to Virgil once more.
I haven't seen all of the episodes that have been aired yet because I don't watch the Japanese ones, so if TR and TP have already been defeated for good or something, then replace them with some other antagonists in this idea.
But either way, I'd name this episode "An Eevee Mystery!" It's kind of a pun on "eerie mystery" but not really. lol