These are all very good suggestions / solutions. +1 to each.

Here's another thought: what if....
the more cities the defending guild controls, the attacker gets a +10% boost (per city).
For example:
-- if dominant guild owns 8 cities (which is very common currently), then challengers get 80% boost (like World Boss bonus).
-- if dominant guild owns 5 cities and challenger owns 1, then challenger gets 50% boost defender gets 10% boost. etc

(in a way it's like the Champ War, sorta)