It’s simple, yet a bit more sensical than what we have now: In any turn you wish to attack strict neutrals, you have the option of “buying off” any remaining ones so that they stay neutral.
Case in point: Germany wishes to attack Sweden, but obviously doesn’t want to turn Spain or Turkey against them. To keep them as strict neutrals, Germany must pay 2 + 2 (the IPC values of each territory).
If it wants to include Afg. and Saudi Arabia, then 2 + 2 + 2 + (say a min of 1 IPC, for those territories with no IPC values).
These territories would also stay strictly neutral in the event of an Allied attack (although perhaps that can be debatable). Also, the IPC costs themselves: perhaps double the tt value would be better, but those are, of course, details.