Sounds like the Villians theme is pretty likely for California Screamin' as part of the DCA overhaul, but here are a couple other ideas I had for adding more theme and story to the coaster. Note that I actually like the coaster the way it is, and would not mind if it stays that way, (unlike pretty much everything else in Paradise Pier )
1) Mickey, Minnie, Donald, Daisy, and maybe Goofy and his son Max ride a roller coaster. Going to a pierside carnival seems like the kind of thing that Mickey & Minnie or Donald & Daisy would do on a date in the old cartoons, and there is actually an old Goofy cartoon where he winds up going down a pierside coaster on waterskis.
From the queue, we would see a parallel queue in which the characters are lined up, talking about the ride, all trying to act like they're not afraid to ride, but sounding like they really are. The cars the characters ride in would travel parallel to ours - perhaps not for the whole length of the ride, but we would see and hear their reactions to the launch, drops, loop, etc.
2) Bring a "World's Fair" element to the pier (I have no idea if there's ever been a World's Fair in CA, but what the heck at this point ).
The roller coaster has been taken over by a scientist conducting experiments in electromagnetic propulsion, using the coaster as an example to visitors of how his technology will work (explaining how the modern launch system exists in a 1920's pier setting).
The queue would be an exhibit of the scientist's experiments.
Possibly, the scientist would be Donald's uncle, Prof. Ludwig Von Drake, but I don't know how well-known the character is nowadays.
If you have any other ideas, please add them here .