The Halloween stuff is totally rad, but they don't have to stick with seasons or holidays. Even just changing the banners on the Tower for Iron Banner is a nice touch. Just change up the Tower in general from time to time. Make it feel like a living environment. Add or remove junk from the Cryptarch's tables. Move the damn broombot. This also brings in the opportunity to do more events. Have an emissary from the Reef show up every now and then, throw out a few bounties, just like what's been done before, and change up the Tower for that week.