I have a couple of suggestions based on the historical Washington Naval Treaty:
1. Allow rebuild of ships under construction to bring them under treaty compliance. This could be reducing the gun size, or carrier conversion.
2. Allow scrapping of existing ships to allow completion of treaty-compliant ships under construction (assuming under tonnage requirements.)
3. Allow replacement clause (ex. capital ships over 20 years old replaced with treaty-compliant ships with the same or less total tonnage.
4. Designation of a ship as a museum ship rather than being scrapped.
To my mind, #1 is the big one, and I would also like to see this included in the game in general. There are enough instances where ships being built had design changes or even CV conversions performed during construction.