I think holding on to the premise that USF has any unifying theme (with the exception of Diagon)  is holding on to the past.   A first-timer would view the park as just big one fun zone; a celebration of entertainment and pop culture with different experiences layered on top and next to each other.   So in that regard, I'm fine with a coaster bursting out of a firehouse window.  They made the decision to do that because it was ridiculous and fun, and I'm fine with that. 
Sorry, don't mean to venture off-topic.