NVIDIA has been on a roll with product launches lately, with the new GTX 1080 Ti and Titan Xp launches, and it would seem the company isn't putting on the brakes just yet. NVIDIA Italy, through its Facebook page, has put up a teaser which points towards April 29th as being the day of a new showcase or announcement - the nature of which is as yet unclear. However, I think it's safe to rule out a new high-end GPU (rest easy, early Titan Xp users, it's unlikely NVIDIA would supersede their highest-performance GPU so soon after launch... Again.)
A Volta announcement is unlikely at this time, at least on the consumer side of the equation; however, the door is open towards the workstation and server side of the equation. NVIDIA does have other lines of consumer products that aren't graphics cards, though, so a Shield announcement, or even a new materialization of the company's expertise in a new product, are possible.
Source: NVIDIA Teases Something New, Announcement on April 29th | techPowerUp
