I'd like to use the Maxis Downtown map to make a custom hood but it doesn't seem to be included in the empty terrains they shipped with the games. I've been searching for a while and haven't found anyone who's unlocked or recreated the map. Or is it already in the game but labeled funny and I'm just not noticing it?

Gah, you know when you do something complicated, but then you can't remember how you did it or even where you saved it? Yeah, that's me right now.

But, more relevant to you, I just answered this question for myself in regards to Pleasantview, because I wanted to edit the map in SC4 and I wanted a copy of the original hood to work with. No idea what I did with my backups but, long story short, you should be able to copy the files from your Programs Files.

So, if you want an empty map of Downtown, go to Program Files (x86)\EA GAMES\The Sims 2 Nightlife\TSData\Res\NeighborhoodTemplate\D001 (or whereever your Nightlife is installed) and copy, do not move, D001_Neighborhood.png and place the copy in your The Sims 2\SC4Terrains folder (in Documents).

If you want the map with the hood deco and buildings--then you can download it as a main hood from meetme2theriver here. Just note:
1. I've no idea if there's a more updated version; I don't keep up with the release of clean/empty templates. This is the one I've got; you can search around to see if there's more recent ones.
2. If you install Downtown as a main hood, it will not function as a Downtown--no Tricous or vampires or downtownies, etc.
3. Sometimes when downloading Meetme2theriver's templates, you may have a hard time with your antivirus program. Mine for example, Norton, constantly flags some of these files and removes them from my computer. Do not be alarmed--this can happen with files that are infrequently downloaded. In my experience, I don't have any problems with the files. I turn off Norton, download the files, then turn Norton back on and run the scanner on the files--Norton always turns up nothing, so I take this to mean that it just flags infrequent downloads.

Maybe you're comfortable doing that, maybe not--you can make the decision to download for yourself, but I did want to mention it since it's happened several times with some of the files. I don't remember if Downtown specifically gave me problems, but again, warning you just in case.

You can get an empty downtown for the mainhood from Jawusa