While my first guess would be shining spears or some new Ynnari unit, I think it is more fun to massively over guess.
I'm going to wildly guess that the image is the top of a massive war bow that uses some sort of 'hard light' bow string that connects the two ends of the bow.
The bow is going to be Kurnous' bow and is wielded by an avatar of Kurnous himself who is going to be discovered and resurrected by Eldrad.
This is going to be part of the sequence of events on the chain to the eventual splitting of Slannesh where part of Asuryan is pulled out of her gut.
The Craftworld Eldar will actually become the Asuryani, and Slannesh is going to be re-imaged as a slightly more kid friendly god of perfection, pride, and arrogance.
Likely none of this is even close to the truth!