Man, this new Gen...
When I first played through X, I committed myself to only catching and raising new Pokemon that debuted in Gen 6. I got pretty frustrated when I was forced to use Mega Lucario in my playthrough. I boxed him afterwards, and I got over it pretty quickly. After beating the game, I was disappointed in learning only old Pokemon had Megas so I didn't have the luxury to use Mega Pokemon in my playthrough. It would make an easy playthrough much easier, but it would've been fun to use though.
Now with Alola forms of old Pokes happening this Gen, I'm really considering on breaking my rules a bit and catching the Alola versions because they could be considered as new Pokemon with new/different typings and possibly different stats. Might do a pseudo-Nuzlocke in just catching 1 new Gen 7/Alola version Poke in each route despite me liking it or not.