They've already torn it down. What attraction screams California (and no, California Screamin' doesn't scream California). Soarin' doesn't anymore, Tower of Terror is gone, Cars Land is a loose tie at best. The only thing that really screams California are the Red Car trolleys.Why? Theme is the glue that holds the park together, without any, it's just an incoherent collection of lands/ attractions. That's the same situation Epcot faces*.
I understand that many people, who have a much stronger attachment to DL, want California Adventure to be the dumping ground for all ideas that don't naturally fit Disneyland (Marvel Land, Star Wars Land, etc.). But DCA deserves to be more than that. DCA is a theme park, not just a high-quality amusement park.
I would say there's plenty California left, at least aesthetically. There's still Buena Vista street, Hollywood, Route 66, a pier, Yosemite/Redwood, etc. I think it'd be far more logical to build on that, even loosely, than to abandon it.
But if you disagree I'd be glad to hear why.
*Yeah, I just related our off-topic drift to back to the topic!
I absolutely agree that you need a cohesive theme to tie it together. It's something that Epcot needs as well and it's why parks like DisneySea and the Animal Kingdom are so amazing. If they didn't do a half #$%&! explanation of the evolution of Future World and Epcot there might be more people on board with some of these changes.
You want to put Guardians of the Galaxy in Epcot? Fine. Do it right. (Disney, if you're listening, I'm available for hire).
I said on here long before Disney made it official that Peter Quill could have gone to EPCOT Center. As a framing device, that absolutely works. But they can't make it lip service, they need to sell you on that idea. So here's one way to do it (not the only way, just one way).
In Avengers Infinity War, it's safe to assume that the Guardians of the Galaxy will have some interaction with Doctor Strange. We know that Doctor Strange has access to the time stone and can therefore manipulate time. Therefore, as part of the attraction, the pre-show, and/or the post-show, you can have Doctor Strange help Peter Quill visit classic Epcot attractions.
You have the main attraction be something for the masses with maybe a cursory tour of Epcot through the years with Doctor Strange changing the environment around you (or perhaps Peter or Rocket takes the stone from Doctor Strange and they keep dropping it or some other form of comedic exchange). In the post show, you can utilize the CAVE, DISH, or some other system to virtual experience EPCOT Center in the past.
That gives you the time travel framing device and motivation from an 80s child that wants to see it, all while using a modern intellectual property.
Now if you'll excuse me, I'll be waiting by my phone.