What if we increase the penalty for non-native troops? (it's -1 now, let's make it -2)?
What we get from this:
1. Increased value of native troops.
2. Increased value of artifacts that boost morale.
3. Reduced profit for Cons, Dungeons.
4. Reduced profit from necromancy for all castles (-3 morale for having necromancers in the army - that's harsh=)).
5. Strengthening of Fortress and Necropolis in contrast, as they don't suffer much from this; we can come up with something else for them.