Some new quests for finishing the final wave?

[Yvain]

Though the problem would be that people would just barrel the majority of the final wave and that would count as beating normal/hard mode.

It is not a problem in my opinion. It's good to add a reward for each normal or hard whole run. Faction points and... I would suggest something certain and valuable (not a quest), e.g. a phoenix feather no, a relic fragment or orichalcum a scale er... 15k ? ^^
(should depend on the mode N/H of course)

And to finish the waven why not use all barrel left ?  This will be funny as a bonus.
But last wave can drop a few things iirc. (nothing precious though.. maybe increase 20th wave drops ?)

EDIT : The aim of the game can be killing the final boss, but why not set this side aim ?