My two cents.
To answer the original question: yes.
Someone mentioned OG San Vanelona. I think that that would be a perfect freeskate area. Work would be required on your part, like retexturing and updating some of the visuals for OldTown and Downtown (if you wanted to make it look like Skate 2, which wouldn't be a bad thing) but it'd be easier than creating the entire thing from scratch.
As for more levels/areas....here's an idea; look to the older Tony Hawk games for guidance. Seriously. I know it sounds stupid, but the early games have covered a lot of iconic areas - the first had San Francisco and Burnside, THPS2 had Love Park and FDR, the Brooklyn Banks (which I saw were included in Skate 3), Venice Beach, THPS4 had Kona...I'm sure that you guys already have plans for your DLC content, but there's a lot of inspiration to be found in those old games. You guys can really take what they tried to do with limited hardware and make it shine.
So, again, the answer is yes. The true strengths of this game come from the skateboarding, not the career mode. If you're really set on challenges, you can make them online-only, which will help the whole "skate online" scene that Skate 3 seems to be going for.