I'm a CA girl, but I have visited all over the US. I like the historical places, but I vote that you visit the natural locations. Since you've already visited the Grand Canyon, I urge you to go to all the red rock places...Bryce & Zion, etc.
You MUST drive along the CA coast (Big Sur and north) and see a redwood tree (either Big Sur, Big Basin, Muirwood, Avenue of the Giants). Yosemite is very different than Yellowstone. Not much can compare to the magnificence of Yosemite, but it is out of the way, up in the Sierras (which are the BEST..anywhere....in the world). I do really enjoy Yellowstone too.....the wildlife as well as the scenery.
You have some really tough choices to make. We live in a fabulous country, too large to cover in six months. Whatever you can see will be a blessing to you and your family.