I think we should stick with confirmed Mega Evolutions; Vileplume lacks one so I don't think we should make one up just for the sake of plot. However, Mega Venusaur would probably serve the same purpose, while also being an existing Mega.
That said, I think I'm gonna start having my team raise a Larvitar so I can go Mega too. Which means my current team, adding in some evolutions that happened during the search for Sableye (IT WAS THREE YEARS!) would be Frogadier, Doublade, Pangoro, Pyroar, Pidgeot, and Larvitar/Pupitar (not sure if I'll bring him in fresh or pull a partially trained one out of the PC like I did for Pidgeot).
Also I thought we were going to only be in Hoenn for a short while before proceeding to Kalos. I suppose once the Team Flora plot is wrapped up that can happen, and we could always use your trainer and mine as a reason. You know, both of us returning home or something.