I don't know that this would "fix" co-op, but it is an irritation I have had for a long while with the game. Blizzard makes games with extreme polish (regardless of whether the mechanics work as well as anticipated) and one of those factors is this: all elements of the game are meaningful and useful to the overall play of the game.
In D3, there are a number of factors that simply are thrown in and have no meaning. Someone on a different post mentioned how the vendors are useless other than to dump your trash items. The thing that irritates me the most is the pathetic experience rewards given for the kill streak, environmental kills, and multiple kills in a single shot. Seriously!!? WTH... the bonus experience you get is often times less than a single monster kill. Of course it scales, but very poorly. Additionally, Who Cares!!? Experience at a late stage in the game is nearly useless, especially dished out in such small amounts. It feels like this aspect of the game was thrown together and is particularly meaningless to how people play.
Make this a meaningful part of the game. Maybe make the reward a small increase to something like % increase experience, gold find, magic find, defense, offense, speed, ANYTHING, Something that has some sort of value. Let the bonus scale similar to how it does right now with the worthless experience. It could be something like 1/10% increase experience per enemy killed for a set amount of time, or even a scaled timeframe that increases with the number of kills. Maybe it would be refreshable or stackable.
This could be incorporated into a co-op game more significantly than single player. I've been in co-op games that the kills go on and on which doesn't typically happen in single player. This could be useful in differentiating co-op from single player. Yes, co-op is harder, but the potential damage buff would level the field, maybe make it easier, quicker, and more preferable than single player. Of course the buff would not be guaranteed, the whole group would have to work together to achieve it.
I'm only one person brainstorming. I'm sure people out there could significantly improve on this idea. But please, please, PLEASE!, fix this wasted gameplay element somehow. It is irritating to me because it could be so much better. On that note, fix all the useless elements in the game. There are more than just this one. It is not like Blizzard to have these in games.
**hopefully this hasn't been discussed before. I have never seen it and I apologize if I'm being redundant.
Edited by Naxan#1356 on 3/6/2013 8:59 PM PST