Do whatever you want to spice up the game, by all means. If you want to change what victory cities there are, change them. Change the victory conditions. Play until you have a certain number of territories, a certain level of combined income, or even until one power can build 10 battleships. If you want to spice it up, that would be a good way to do it.
But most people, in my experience, don’t pay any attention to victory cities. Just play until one side gives up. Usually this happens before 15 victory cities are obtained.
It does not matter that Washington, Ottawa, and San Francisco are hard to take. If you’re playing the standard 15 victory cities, you don’t need any of them. If you’ve taken over Moscow and London, it is only a matter of time until all 18 cities are yours, so it really doesn’t matter.