Can I make custom arenas? I want a large one that incorporates cars 
i think there are plans for scenarios when the mod manager is done. would be curious if its possible to do this now.
Interesting idea. The way it works right now is defense mode spawns one of the predefined buildings (megastore, bar, hospital, or mansion), it’s certainly possible to make new defense-mode specific buildings.
There are discussions of adding new buildings to the game in several threads, such as http://smf.cataclysmdda.com/index.php?topic=941.0, and vache is working on a standalone building editor that will let you layout new buildings. It outputs the source code that needs to be patched into the game to include the building.
one fun one would be a very good shelter with lots of traps and such around it and borded windows. good weapons inside and explosives.
then 10,000 zombies coming right at you.