If you were the Wine King of California and were tasked with dictating
rules about which grape variety could be grown in different regions
how would you lay down the law?

In most area's of Europe it's hard to get away from the notion of one or two grapes defining a particular growing region. They have laws that say you must use particular grapes to put the place name on the label. In America it's different.
We put varietals on our labels here. People don't buy a "Napa Valley" and necessarily know what they are getting. They buy a Cabernet and that's what they get.
But this doesn't mean that various wine regions and appellation associations around California don't try, and try hard, to associate themselves with a particular varietal. Some areas become associated with certain varietals by default too.
How would you lay down the grape laws if you were Wine King? You may choose no more than three grapes allowed per region. Here's one fake king's response:
Anderson Valley:
Pinot is grape for which this cool region is best known, and for good reason. They locals have worked hard to promote their Pinot, but the competition for top Pinot region is tough. What they should and are doing is push equally hard for Anderson Valley to be known for Alsatian and German varietals such as Pinot Gris, Gewurztraminer, and Riesling. Without question the Anderson Valley is making the best examples of these varietals in California. Problem is, you usually don't get as much for a bottle of Gewurztraminer as you do Pinot.
RULING: Pinot Noir, Gewuztraminer, Riesling
Dry Creek Valley.
They make a lot of great wines in this northern Sonoma appellation but clearly Zinfandel is king of the hill and the varietal most associated with the region. It seemed at one time that Sauvignon Blanc would become associated with Dry Creek Valley. There are still great versions made, but SB can't top Zin. And by the way, who wouldn't want to see an annual Dry Creek Valley Zinfandel Conference?
RULING: Zinfandel and Sauvignon Blanc
Napa Valley
Everything in Napa is about Cabernet. Yes, yes. I know...lovely wines of all sorts. But Cabernet and Napa Valley are about as closely associated as Mercedes is with Benz. Interestingly, the Napa Valley Vintners have not pushed Cab as the grape of Napa Valley.
RULING: Cabernet Sauvignon, Merlot, Cabernet Franc (some wineries secretly plant Malbec and Petit Verdot)
Russian River Valley
Clearly this large Sonoma County appellation is best known for Pinot Noir. In fact, you can make a pretty compelling argument that it is the best known Pinot Noir appellation in the world after Burgundy. Yet, the size and diversity of what is known as the Russian River Valley appellation means you've got a number of other varietals that create wines that are at the same high caliber as their Pinots. I'm thinking of Zinfandel, Chardonnay, and Petite Sirah in particular. The key to understanding Russian River Valley is understanding it's subsections. Green Valley, for example is really extraordinary Pinot country. The part of the appellation east of highway 101 is all about Zinfandel in my opinion.
RULING: Pinot Noir, Zinfandel, Chardonnay
Alexander Valley
Here it's mainly about Cabernet. This warm region of Sonoma County is home to the likes of Silver Oak and Jordan, two Cabernets that really helped put Sonoma County on the Cabernet map. You'll see other varietals such as Merlot, Zinfandel, Sauvignon Blanc and Syrah. But if you have to pin one varietal on the region, it has to be Cabernet
RULING: Cabernet Sauvignon, Syrah, Chardonnay
Sierra Foothills
This stunningly beautiful region doesn't get as much traffic as Sonoma and Napa. But if it did, you'd have more people appreciating its Zinfandel and Sauvignon Blanc. But because there is not nearly as much wine being grown here as in other areas, you don't see it associated as often with these varietals.
RULING: Zinfandel, Sauvignon Blanc
Santa Barbara County
Pinot Noir (Thank you, "Sideways") Even before "Sideways" made its impact this region was becoming stiff competition for Russian River Valley. The movie and the collection of artistic winemakers is making that competition. even stiffer.
RULING: Pinot Noir, Syrah, Chardonnay
Monterey County
Monterey has had its ups and downs. It's a large region that originally planted huge swaths of land to Cabernet. Of course it was too cool and the region got a reputation for producing herbish tasting cab. Today the region does not have a reputation for any particular varietal. However, give it a few year and Monterey and Pinot Noir will be closely associated. There are some marvelous and serious small vintners chasing lots of Pinot in this area as well as some top notch Pinot vineyards.
RULING: Pinot Noir, Chardonnay
Carneros
Again, Pinot Noir...But also Chardonnay. Carnerso is one of the original "pinot regions" and with good reason. Acacia, Carneros Creek, Saintsbury, Schug all began here with a real belief in the areas potential to grow outstanding Pinot. And they've proven this. Site selection is really key here.
RULING: Pinot Noir, Merlot, Chardonnay
Howell Mountain, Spring Mountain, Mount Veeder
All the mountains surrounding Napa Valley are or should be associated with Cabernet. It's very special fruit.
RULING: Cabernet Sauvignon
Santa Cruz Mountains
Probably the best kept secret in California. Most would say that this region is all about Cabernet. And how could it not be. When you consider that a number of outstanding Caber nets come out of these hills you have no choice. Hell, the Monto Bello vineyard's existence in this area is enough to peg is a a Cabernet region.
RULING: Cabernet Sauvignon, Chardonnay
Sonoma Coast
Yes, I know, Pinot Noir. But the problem is that his "appellation" is so gigantic and diverse that you really can't say that the region as a whole is suited to any one appellation.
RULING: Free for all
Sonoma Valley
Perhaps the best know wine region after Napa Valley, yet no one varietal has become associate with this long valley to the west of Napa
RULING: Zinfandel, Cabernet Sauvignon, Any White
Atlas Peak
Here you have an appellation that was once known for being a source of average Sangiovese. That reputation is slowly going away as more and more wineries produce Cabernet from the hilltop region. Give it five years and it will be all about Cabernet
RULING: Cabernet, Syrah, Chardonnay
Chalk Hill
You could argue that the region is associated with Chardonnay if only due to the fine Chardonnays that have been produced by Chalk Hill Winery. However, so little has been produced, by so few wineries, there just isn't enough to justify any particular association.
RULING: Chardonnay and Cabernet Sauvignon
Temecula
The sorry thing is that this Southern California region is mainly associated with a flying insect that devastated its vineyards. It really hasn't distinguished itself with an association with any primary grape.
RULING: I have no idea!
If I were king.....