I agree that the megabasing concept can be a bit vague but it's because not all spots on the map are the same and it's more of a common sense thing to not take over an entire district. Just think about it as a larger base made out of props that incorporates multiple individual bases or a large part within a district.
I think and i suggested in the meeting that the rules should be clarified around megabases that to defend them you must be in the party of the owner. This is because it is almost impossible to make a megabase where a single front door is visible and it doesn't make sense for every single member of the megabase to be on that one front door.