[@Willy Vereb]

It's a lot of map work to go through and find all the tiny unimportant pinpricks on the map that would go to Prussia. Germany is a mess enough as it then to worry about Free Cities.

A lore rationale may just be as simple as the allied nations needing a way to contain France at the Rhine since a multiplicity of small states or even a medium sized German state wouldn't be able to contain the French. As the Congress of Vienna the goal of the Congress of London is to go, "This second time around, France is the culprit" and the Napoleonic Territories built on the East Bank of the Rhine need to be deposed and their territory deconstructed and reconstructed. Appealing to much too Austria too would also mean surrendering Central Europe to Austria and neo-HRE visions for Germany, which also can't happen. Birthing a new Prussia is to secure the Rhine vs France and keep the scales balanced vs Austria while also working with Austria to manage all the Germans despite whatever opposed goals they have.

How it is they achieve that and what the full meaning of that is is a mystery box because even at the IRL Congress of Vienna as at the Congress of London Prussia would be a massive mystery box that has to exist because if it didn't it'll be round three all over again if the Bourbons fuck it up again. The Prussian State may protest, knowing their own Ancien power base is crippled, and any of the minor nobles who got negotiated into it by Talleyrand are just there to be secured vs France.