I would absolutely love Disney to have another private island/beach to visit. We really liked RCCL Labadee when we visited, even though it isn't an island. You still get that island feel and it is still private to guests of the cruise line. I like Labadee also because they have a dock. I'm not a huge fan of tendering.
Which brings me to Catalina. There is no dock large enough for cruise lines there and the coastline does not lend itself to building one. It's a tender port. That said, it's a lot of fun to visit, whether from aboard a cruise ship, taking one of the boats from the California coast or even on a private boat. There is a lot of history there on Catalina and I recommend visiting the Casino there and taking the tour. Don't think it is for gambling either. You won't find a slot machine or gambling hall anywhere in the place.
As for West Coast temperatures compared to Caribbean. It's pretty clear that the Caribbean has warmer overall temperatures, but depending on the weather patterns, don't be surprised if the water feels cold on some days when visit Castaway Cay. The Pacific Ocean is generally cooler, but it's never bothered me. My wife is a more warm water person though, so tends to steer clear of anything in the 70s.
Finally, Disney has not found a way to make the Mexican Rivera profitable. They tried to leave the Wonder on the West Coast for an extended stay, but it just didn't work out. They experimented with the Pixar cruises and it drew passengers that eventually led to Marvel Day and Star Wars Day, but they still have trouble filling the ship for the Mexican Rivera. There is no way they would have a West Coast private island until they could prove the West Coast is a profitable area for cruising.
Interestingly enough, Carnival is making a big bet on the Mexican Riviera and Port of Long Beach. Major port renovations and it's newest ship being deployed there. Will be fun to watch what happens.
Once the new ships come out, Disney may want to leave the Wonder on the West Coast, but the question is, where would it home port and what would it do? They could heading to the South Pacific after Alaska and maybe even as far as Australia, New Zealand or the Philippines. I can totally see Disney heading from Vancouver to Hawaii then on to Fiji or French Polynesia.