Sounds like Special Events where some Maps are harder because of special requirements like:
-Units spawned are always Dazed (Half Life) through the entire run
- Enemy Units have 50% more HP or something like that.
-Any other handicap
Completing them give more rewards or a special Reward.
Would make the game more interesting. Its like Map of the Month where it is Monthly or weekly events.
I think thats a good idea