One suggestion I'd have is being able to play as your own rebel group.

-You Pick a name and flag
-You are only allowed to use insurgent troops, but you could somehow "capture" army equipment from the country you are fighting against
-The territory you control will have a production reduction
-You can only fight one country, or you can ally with one country and launch attacks from that country
-You can sue for peace, or if you take over the country choose a tech tree

Possible Cons
-Wolf packs on specific players
-Too many groups; People start only playing as rebels

What do you all think? If this has already been covered, I'll take it down. Maybe you pay gold to be an insurgent?