Leave your game open enough so that you can add new armors, weapons, skins, vehicles, and classes AFTER the game launches.
I remember far too many "Awww man! That would be so cool to have in MP! Too bad we can't...." moments for the Halo series.
If you could also learn to Show and not Tell that would be fantastic too. (Reach)
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Of bungie is going to learn anything from Halo is should be to nice sell Destiny to a company like 343i