What I want from a mega flygon,like others,probably involves a change in typing or a boost in speed,which an alt form could accomplish. Like, an alolan flygon that's bug/dragon, or an alolan meganium with grass/fire typing, would be preferred to giving them a mega design,which is restricted to the original typing in some way and lacks the ability to hold an item,and prevents you from using other megas(the ability to use only one of the numerous mons they have improved is a bit frustrating :/)
Lower tier megas are in the minority though! Most of them can sweep teams in singles,like the '-ate' ability nukes,Hugepower+Huge attack stat,parental bond...
even mega banette,priority destiny bond is a bit much,wouldn't you think? I love mega banette,but that's a very cheap way to KO something.Granted,not as bad as M-Gengar's perish trapping,but still,I feel guilt when using something that has been designed to be better than 'standard' pokes