They've tried that. 2008 had Fractured Tales, a zone in KidZone proper near what is now Spongebob Storepants and the ET entrance. As you might expect, it got extremely congested. Similar thing happened when they tried to move the zone into the cul-de-sac in front of Animal Actors the next year with Cirque Du Freak. Neat zone, but packed to the gills (similar to the more recent San Francisco zones).
Back when they were doing hordes, I also noticed the actors situated at Central Park would wander into the side trails closer to the lagoon, too. That wouldn't really be possible now since it's the viewing area for the lagoon shows.
But yes, I could totally see a scenario where we have zones in the following areas --
- Plaza of the Stars (Themed/Marked)
- Main New York Streets, Near Fallon (Themed/Marked)
- San Francisco (Themed/Marked)
- Hollywood Boulevard (Themed/Marked)
- Central Park (Themed/Marked)
- Front Park Plaza, Near Hello Kitty/Studio Store (Horde Only/Unmarked)
- Side New York Streets, Near Lagoon (Horde Only/Unmarked)
- Springfield (Horde Only/Unmarked)
- World Expo, Historically Near MIB Store (Horde Only/Unmarked)
Attempting to use London has gotten them into trouble and I believe is considered part of Potter with the Knight Bus. Sting Alley is also, unfortunately, no longer practical.
For those worried about budget, the hordes are usually very cheap generics or costumes they have sitting around in storage. They're an easy way to fill out the park, and actors in the hordes can be pulled to fill holes in mazes and the more prominent zones.