Idea 1:
They should not send just 1 unit and attack uselessly well-fortified cities. Make them more clever: let them destroy roads and capture workers and choose easier targets. Causing trouble is better than death for nothing. Also they should wait to gather at least 2-3 units to make a raid.

Idea 2:
Get back barbarian leaders we could catch and add an option to release him for ransom or execute to intimidate the bustards.

Idea 3:
Barbarian unit weaker than regular is not fair. They should be at least equal to regular in strength. Remember that some barbarian tribes were very skilled worriors.

Idea 4:
Add occational population boom. Barbarian hords should be able to sweep entire realms.