I would like to contribute some of my enhancement/patch ideas to D3. Not sure if this is something Blizz developers or fellow gamers agree, but I will give it a shot :)
Coliseum of Caldeum
In Act 2, design a door where single/a group of players can enter. This is some sort of battleground stage where gladiators fight with monsters and other gladiators. An example of such stage is the Magdha boss fight area. In the coliseum, players will be fighting AI controlled Dark Nephalems. So the Dark Nephalem is something like the dark self a player encountered while fighting Diablo in the dream world. In this case, Blizz can make use of the dark self found only during Diablo fights in this area.
The Dark Nephalem will be using a list of spells/skills similar to player class. So this creates a different kind of fights, almost like PvP.
For reward, we can still use the normal NV, or we can try something different. For example, for each completed battle, 2 or 3 items weared by each Dark Nephalem will drop. These can range from blue to yellow to legendary to set item.
For more interesting fights, we can allow for some randomization in the creatures generated. For example, in one fight there can be 1 Dark Nephalem, 3 elites, while another fight we get 4 Dark Nephalems. The idea is to keep the fights varied and interesting.
Or instead of Dark Nephalems, use randomly generate AI controlled gladiators. These can range from follower to player classes.
Have a nice day.