I don't like the idea as well for the current setting... I mean it might be original, it is still impossible to get some small strategy with your teammate, or will never know when you need to go and save him... It will sound more like a solo where the resources are in common. And that's it.
Furthermore I don't understand why it is so impossible to sync all those datas. If I take Diablo 2, which is a game from 10 years back, I used to play on Battle Net. It was a 2D game (isometric to be precise), and you could team up with other people. There were sometimes a lot of ennemies, with a lot of particles and effects... But they all were synced. And 10 years from now it was a much slower connection than what we have today... Much lower bandwith and speed...
So I don't quite good understand why it is so impossible to make... I agree that with Diablo 2 (and D3 as well), each game was populated by a max people inside the game. Why not give that kind of limitation ? For D3, everyone shouted that it was a bad idea to limit one game to 4 people playing together. And now with the beta, they all said it was enough and great that way.
Instead of not knowing how much people can join, what about setting up a limitation ? Well, one idea, amongst all the others