How about these options (to be decided by the game host) just to get a bit more varied alliance tactics in the games.
1 as today
2 prealert mode. alliances can be broken only if you first select "break alliance". This can only be done up until the point in time when your allied has its turn so he/she can decide how to deal withe breakup first. Makes it harder to break up.
3 tag team mode. as today but allies can form a joint continent without either of them having all countries. The combined countries can form a continent that favours both. Makes it worthwhile to stay together a bit longer
4 visible alliances. As today but all alliances formed are shown to everyone.
5 crazy mode. The rules will change every 30 minutes from the above variants.