I've been working on getting all of California's historical county borders onto the VT-BAM, and I decided to map some of the proposals for the division of California at the same time (since most just use county borders, or a simple straight line on some latitude). Here are the two most interesting/important (in my opinion). If you want me to put some more in here, I can.
The Territory of Colorado, as it was officially voted on and accepted by the government of California in 1859. (it was pretty much just ignored by the US congress).
The plan to enlarge and then trisect the state of California in 1855. I assumed that the horizontal straight line borders of the eastern counties would simply be continued to the new eastern end of the states, as that makes the most sense to me. The states would be called Shasta in the north, California in the middle, and Colorado in the south. It is noted that there was generally little opposition to this proposal, but many people believed that it would likely be unconstitutional. As with the later proposal from above, it was mostly ignored because of more pressing political issues in the US government.