So I didn't read an the comments, so I'm sorry if this has already been said.
I like the idea that Unity is important, but I don't think that it's just another shard
I really like the idea that the Dawnshards were used to kill Adonalsium by removing Unity.
I think that Unity is the 17th shard. Kind of. Apparently, when you take a piece of Adonalsium, it gains an intent and can kind if do a few things on it's own. (Preservation was able to choose Vin as a vessel while not correctly holding a vessel)
If Unity was broken off of Adonalsium by the Dawnshards, it is probably weak, drifting around the Cosmere, and without a vessel. However, like Preservation, it may be able to temporarily inhabit a vessel that fits it's specifications. Like Dalinar while he was fighting Odium.
This could also explain some things like the presence of the in-world organization of the 17th Shard, who may have heard of this Shard's existence, and the perpendicularity on First of the Sun.