... It's all of the default Maxis hoods joined by the mother of all time paradoxes...
I have a feeling you're idea of what the MegaHood is, is wrong. It's not some enormous Hood, with the land/lot space of all hoods combined. It's all of the sims from all of the Hoods combined into one normal Neighborhood. Unless I'm totally off on what it is.
More sub-hoods does sound like the correct form of action here, by creating multiple subhoods in each category. I wouldn't do exact copies of Downtown and Bluewater (as one doesn't need multiple copies of some of the sims and/or houses placed), but there are several empty pre-made varieties to choose from or download other maps that modders have made.
You must not be much of a Sci-Fi fan. The statement "Mother of all paradoxes" was to imply that by default, neither neighborhood "exists" within the same time frame. Seeing as you can't take one sim and plop them into the next one without crashing your game, which is akin to taking someone from say, the 25th century and dropping them into 12th Europe with a laser rifle. It was just a figurative comment, not literal. Just because I only have 9 posts now doesn't mean I don't know the game works. Only an idiot would thing the Megahood actually adds more real-estate to an existing map.