I'm not sure if this has been suggested, but there should be an
indicator of whether a Pokémon is Mega-Able at the Auction, GTS
Setups and GTS offers, as we can't tell whether a Pokémon is
Mega-Able without checking its individual profile. After all, there
is an indicator for Shiny Pokémon, plus their sprite looks
different, but Mega-Able Pokémon look exactly the same as their
normal counterparts. This would be good for people with messy boxes
(like me, boxes are pretty expensive). The bubble could be beside
the Pokémon's level.