So we have an interesting proposition here, most mobs are predetermined at a certain level, but we have some areas that they could be put into fairly easily. For example we have 2 bat caverns. one near the pirate base and another near the boken coast. In this case I would say the second bat cave (boken coast) could host a higher level bat. In this example 17. because no mobs exist around level 17, but we have level 15 bandits and level 10 bats. So you could also argue for level 12 bats. Why a higher level of bats? just to generate a little more experience as players are beating them down for achievements or gear.
High level Pirates/Marines mobs. These could fit in on the sides of Marine Fort to help defend the flags. furthermore when the rocks are attacked, mobs could spawn to help keep players away from the rocks. groups of 5-6 Pirates/Marines at a time.
The invasion event mobs could also be reflected in a similar manner, they have an experience boost they could also have more of their own experience.
Leopards could be prowling the Boken Coast between the two ledges at a higher level too.
P.S. Please consider Taco/Tuco Tuesday, where tuco has a twin with him all day (only one of them have a chest!)