Battle Stadium Head
The thing is, we don't add things to this list based on theory / ideas. Mega Blastoise hasn't shown up in a single one of our tournaments, it floats between 110-120th in usage on the cartridge ladder, and has only been used in 7 high-ranking Japanese blog teams - none of which had a Mimikyu and were all in the SM meta (before USUM changes to the meta). Blastoise is completely unproven so it's not worth adding to the ranks.I still believe the Mega Blastoise - Mimikyu core I alluded to offers Mega Blastoise its niche in BSS the would warrant some sort of C ranking. Going down the list of Mimikyu's answers; it nets a positive matchup vs most steel types that wall Mimikyu (including the aforementioned Ferrothorn), Lando-T and Hippowdon, Porygon2, Cloyster (who sets up on Mimikyu and breaks the Disguise with Icicle Spear), and Mega Gyarados (Mold Breaker bypasses Disguise). I won't say any more on this for now, but I might build a new team around that once college work dies down and post replays of it. Although I do agree that C-Rank could be expanded.
I would also vouch for Reuniclus to C-Rank.