Right now, XP is only used for the cash shop. If you (Bungie) could bring back the system where nodes on a weapon or armor piece could only be unlocked after gaining a certain amount of XP with that item equipped, not only would XP be useful again but we would have something to grind for! You could treat all of our current items as new/unupgraded for the purposes of implementing an update. To be quite honest, I’m not suprised I don’t see more people talking about this. We would have to work for our weapons again if this system was brought back.
This is just bringing back the shit part of the grind though. Not the reason that made us grind in the first place, which was random rolls and the hope for powerful loot. Just adding grind back by itself isn't how to save Destiny but giving us the incentive to want to grind would go a long way towards fixing it.