When northwest Nevada was part of the Territory of Utah, some of the non-Mormon settlers pushed for annexation by California. The California legislature established Pautah County, encompassing an area from Lake Tahoe to Walker Lake and Pyramid Lake in 1852 or 1853. This was subject to Congress ceding the territory to California. That did not happen and the legislation was repealed in 1859.
The Nevada Territory was established in 1861. This in part reflected the need for better government closer at hand than Salt Lake City – especially after the start of the Comstock – in part the conflicts between the Mormon and non-Mormon settlers, and a general dislike of Mormons at the national level.
Have instead Congress agree to Pautah County and its incorporation in California. Be the boundaries the original or the initial boundaries of the Nevada Territory, you then have conditions under which transfer of the Arizona Strip to the enlarged California would make almost as much sense as the transfer to Nevada.
One of the reasons Nevada attained statehood as early as it did was to secure three electoral votes in the 1864 election, as well as some seats in congress for the Republicans. It turned out to have not been necessary, but that was not known. With much of Nevada part of California, those three votes might need to be created elsewhere.
East Tennessee, analogous to West Virginia, would be a possibility, but the military situation later in the war would have made that difficult. An East Tennessee starting in 1861 would be more plausible.
New Mexico could be an option – it was more populous than Nevada at the time. – but any proposal would face ethnic based objections just as admission of Utah faced religious opposition.
Colorado would be a possibility. With Nebraska another. Their populations were still low.The population in other territories was low enough to make statehood hard to justify. Even with the Comstock the Nevada population was low. The 1870 population was less than half the 1860 population for New Mexico, and the Comstock was still going well in 1870.