Last generation I was pretty successful with a highly rated Japanese Mega Steelix Trick Room team. It packs surprising power; after a Curse it OHKOs Mega Kangaskhan and Garchomp and thanks to Curse you can easily set up on physical attacks, even super-effective ones. The threats are much different now, but as a Steel/Ground type with decent bulk it might be ok at dealing with some of the new threats, like Tapu Koko, Tapu Lele, and Mimikyu. We won't know for sure until we try it but I think it could still be a good niche choice. Cresselia is a bit out of favor now but that can be both a good and bad thing for it.
Oh and I just remembered my banner Pokemon is a Mega Steelix, so maybe I am a bit biased lol. Mega Pidgeot I'm less sure about but with that speed tier and special attack I'm sure someone can find something to do with it.
Man I've been wanting to craft a Mega Steelix team like that since ORAS with no luck. Would there be any potential place I could look up a team like that?