This is intriguing.
So consider this:
Assign an IPC value to each neutral. Factories??
Assign a military starting set up to each neutral i.e. Spain, Sweden, Turkey, …. etc ( I’d like to see some hypthetical values expressed here, Turkey’s the same as the suggested units for Spain?)
Set the units up at the beginning of the game.
Then IPC bids by each nation to control that neutral. Any nation may compete to deny that neutral to that bidder.
Or if there are no bids then the game begins as normal, but invasions of neutrals may take place and the winner takes control of that neutral country, it’s colonies if any, and it’s IPC’s and it’s remaining units.
I like this!