I always thought it'd be sweet to do a rooftop skatepark, maybe with a sweet little mini ramp Bam style. If not that some more night time parks are always awesome, nobody does night time parks for the most part. I also think maybe the following could be cool if attempted;
Sewers
Old abandoned prison with lots of long halls, metal secured benches, basketball court outside, etc.
Jungle themed park, maybe like just pure concrete bowls/trannys like completely surrounded in heavy jungle and bushes.
An abandonded zoo with cages and exhibits, empty bowls for animals, etc.
Best ideas I've got right now.