I played a few risk games like this (can really only be done with 4+ people).
Basically, you have 6 colors.
Red Can only attack Blue.
Blue can only attack Green.
Green can only attack Yellow.
Yellow can only attack Pink.
Pink can only attack Teal.
Teal can only attack Red.
Now, if You're Red, and you kill Blue - You can now attack Green.
It is fun to play because you'll get boxed in by people that you can't attack, but they can attack you, or you both can't attack each other, so you just start building up.