An Enemy wave PVE type instance would be one in which the players fight monsters that keep coming in waves.
What would make this excellent though would be thinking outside the box when it comes to party composition! Include gatherers & crafters!
Battle classes
Take on incoming monsters to try to stop them
Gathering classes
Gather items which go to a "party inventory" (similar to POTD's pomanders)
Crafting classes
Use the party items to make party buffs (boost HP, attack, etc...)
And to "repair" weapons that are on the map.
Example: "Broken Cannon" would need 10 gathered slabs of (instance-specific) iron that the crafters use. Once it's repaired, any player of any class can then use this to shoot enemies for 5 cannonballs before it breaks again.
Rewards
Based on how many waves of monsters before a wipe.
Or based on a timer.