In relation to question about pre-game alliance I was thinking about limitting number of players whoenter map in join effort to win it.
As there is no limits on how much resources you can move between friends and this grately influence starting balance I would say there are two choices:
1. Automatic limiting number of resources/money send to friends including restriction on market so they do not use market to go around. Same with number of troops 1 player can receive from friends per day. Additional limit on taking for free provinces from friends . This is less likely to work so here it goes second idea.
2. very strict rules how people can cooperate with each other and then GOs thake responsibility for point 1 checking how people cooperate with each other. This will include setting up limit up to say 5 people working together on the the 500.
I saw people sucking up 600 provinces belonging to dozen of players in a matter of 2 weeks, taking over resources provinces and mech army. The only reason that peopel were takign friends to a map 500 was to use them later as a "food". When friend was having problems in war, was sometimes going afk, was getting bored or did his part in protecting eastern border this guys were eaten alive without any resistance. Now this is something that grately influence balance of the game but as you see single players have no chance at all in supremacy played like that. This in response force other to create huge pre-game groups up to 15-30 people joining same 500 map. I will say that it is closed circle and devs job will be to break it and replace it with something more balanced and user friendly. Once more- not skils but share number of supporters count in that strategy. And you probably guess that some players from top ranking with great achievements are using this strategy of eating allies.
I would be grateful if smething could be done about that.
For more info please look in this thread "Is it legal for someone to donate all of his provinces and armies to some other player drastically changing the flow of the game?"