I think that including local splitscreen would be a way to strenghten the multiplayer aspect of the game. I've wanted to play co-op with my brother, but we haven't been able to because we only own one PS3. Plus, couch co-op has always been one of the funnest parts of video games! And Borderlands managed to pull Splitscreen in a pretty neat way, I think Destiny can pull it off too.
Thank you for reading!
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How that can be done is basically 'borrowing' another Guardian from the same account to act as Guest, addendum being that the Guest has to follow the main Guardian user. I don't think they'll do it however, so you may have to acquire more funds OR have another Guardian character for your brother to play while you probably Garry's Mod on PC or something.