An 18-inch Hood could break the historical treaty system

How would eight 18-inch guns on Hood affect the interwar naval treaties and later battleships?

It could force an 18-inch category or a 45,000-ton standard, making the later British, American, Japanese, and German fleets fundamentally different.

An 18-inch-gunned Hood could not be treated as merely a slightly larger capital ship at Washington. The United States would demand either its own 18-inch allowance or a separate category limiting how many such ships each navy could retain. A 45,000-ton, 18-inch treaty standard would change Nelson, Rodney, later British construction, and the entire replacement schedule. By the 1930s Germany might emphasize very fast battlecruisers or smaller carriers because its yards could not efficiently match the enlarged Allied battle line.