That would be quite interesting. So this guy would be the ruler there?What about the catholic branch of the Hohenzollern?
Anyway, what would be most interesting IMHO would be a republic.
What would be a good compromise between a Reichsland and some imported ruling family would be annexation to neighbouring states. Thr Prussians get parts of Lorraine with Metz, the Bavarians get Haguenau and Alsace down to Straßburg, Badenia gets the rest. Therefore, the alsaciennes would be integrated into the states proper and would have full rights. Given that Bavaria and Badenia will likely have a more liberal approach that could help as well.
This leaves most of the francophones in Prussia. Could this spark a sort of double kulturkampf?