I'm really glad Bioware took the risk and put in some co-op. Mass Effect could work beautifully with it too. Already companion NPCs interject with various bits of dialogue during interaction scenes. Multiplayer co-op could simply have the companions being controlled by your friends, and the dialogue they interject with is chosen by your friends too, from a dialogue wheel. In this way, the companions seem more "alive" because their dialogue is being actively chosen by a real living person. Plus this increases replay value since you might want to replay the mission and have your friends choose different dialogue wheel options.
Co-op would be complicated to implement if all the players had freedom of movement/exploration during non-mission sequences though. Like exploring the Citadel or something like that. Most people would not want to just tag along behind the Shepherd player in these situations. Anyone who's played Neverwinter Nights 1 or 2 or Baldur's Gate co-op knows that this can be sort of fun, but also a bit weird.
But, if Bioware just makes side co-op missions where Shepherd and his friends are deploying to some battle area... then I don't see where any big problems could come from. If I'm not mistaken, Bioware is already doing this with The Old Republic.
Another idea is to branch this co-op MP idea into a competitive mode, where two sides of players (3 one each side) would have a scenario where all 6 players are choosing dialogue options on their individual wheels. It would be a semi-big feat for the writer to do, but if pulled off, would be really cool IMO.
Personally I really hope story-driven multiplayer gaming becomes the next big thing. I'm tired of multiplayer gaming being all about nonstop killing. I feel like MP gaming hasn't really evolved in at least a decade, if not longer.
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