The number of attacks against a player's base could be related simply
to the number of attacks of this player against other players.
- One attack per week for people who is never attacking the others.
- Each time a player is attacking another, a counter is incremented
so this player can be attacked, with no limit by day,
and his counter decremented, until it reaches 0
(then the rule before applies).
Revenge is not counted as an attack (Talion's law).
This to avoid harrassment of players who don't attack others and
like only to build a base, attack convoys, play missions and events.
And above all, to avoid impunity of players attacking the others a lot
(imagine 5 minutes per attack, plus a few for seeking preys, during one day!)
and being attacked only a few times.
I think this would better balance the game.