It would be nice to revisit old campaigns again. So a rotation system for those should be possible bungie. In that case you could re-add the exotic missions that came with those campaigns as well. And in order to manage the space you could remove one of the exciting ones.... This also could bridge the gap between old- and newlights.
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I think having timeline missions featuring a key aspect of each campaign would be better at this point. For Red War, either Homecoming up until we get our Light back at the shard (to explain why Ghaul was so dangerous and how getting our light back was key to winning against him) or 1AU to actually retiring back to the City and killing Ghaul (to explain why getting rid of the Almighty was so important, plus that moment of sweet victory). For Curse of Osiris, probably the final fight against Panoptes (to explain why it's removal was so important to prevent the Vex's Dark Future) For Warmind, probably the Will of the Thousands mission (to explain why defending Rasputin from Xol was critical).