I'll definitely have to start out at least one day at DCA. Which we often did before Cars Land, but that was usually 3 or 4 days into the trip when we were tired and declared that starting with the park that opened at 9:30/10 AM was a perfectly legitimate strategy.

I would definitely consider using a Magic Morning at DCA instead of DL. If MMs are offered on off days from DL's and on-site hotel guests can get in to MMs at both DCA and DL, the value (and prices) of the hotels will probably go up even more.
Now that I think about it, it might actually take two regular park opening mornings to experience Cars Land without long lines, as I believe Mater's ride and the Flying Saucers are both low-capacity. By the time you got through riding one, I wouldn't be remotely surprised if the other was at a 30 minute wait, and wouldn't be shocked if it was more than that. Will be interesting to see.
I'm not honestly certain how much of a difference Buena Vista Street will make for crowds when it comes down to it--initially, fans will be flooding to see the changes, but in the long run I think the decor will be more of (an amazing) atmosphere than something that will consciously draw people. And while a lot of people love Main Street, a very small percentage (I'd say pretty much the die-hardest of die-hard fans) actually visit the park BECAUSE of Main Street. In fact, I'd wager the overwhelming majority of visitors want to go for the thrill rides and/or kid rides, and see Main Street as a place to shop and watch a parade/fireworks, but otherwise take it as a place you walk through to get to more exciting things. Now, granted, if BVS consciously or unconsciously brings in 20% more visitors, that would make for substantial crowds, but I'm thinking the actual number it brings in (once it's been around a while) will be much lower.
Cars Land will definitely draw substantial crowds, of course. I think DCA will get particularly busy because Disney pushes people to get non-hopper passes now, so a large number of those people who come over for Cars Land will be in the park all day. On the other hand, I am
guessing the vast majority of people with 3-day non-hopper passes will still probably spend 2 of those days at
Disneyland and 1 day at DCA. I'm not sure if DCA will ever be able to replicate the nostalgic appeal of Disneyland, and Disneyland still has a higher capacity and more rides.
But if Cars Land and DCA's extended hours make people spend 2 days at DCA and 1 at DL (or turn that 3-day trip into 4 days, 2 in each park)...then we might almost get to the point where people go to Disneyland as the safety valve when DCA is too busy.

Or both parks could be totally crowded....which Disney would love! Me, not so much.
