tardy coral
For exemple, there could be a pick-up that would let you throw the dagger at your friend. As long as the dagger stays on(in) your friend, there will be a ray that'll appear between both of you every 2-3 second for about 1 second. If you pull it out it won't work anymore until you throw it back on your friend.
You sacrifice your dagger for another type of lesser automatic damage.
This, obviously, would only be pickable in co-op (Or, it could create the beam from you to wherever the dagger is)
austere oasis
I also would enjoy some relics tailored to cooperative play
Maybe some adjustment to existing ones as well, like healer's mark healing both, or something
As it is it feels odd the degree to which you don't really interact with each other
Let me pick up my friend's crossbow bolt for him, let me heal a friend, let me throw a friend, anything
The only thing there is now is you can hit your friend with orbs, or hit the both of you
I wanted to get my co-op partner to stand on my sword xD