Over on Warseer the cover of both magazines has been leaked. This shows that yes, Dwarves are next, and a large Tyranid on the cover of Warhammer Visions. For instance, on the cover it says "How to paint the new Longbeards".
I am not sure I quite get why Tyranids are on the cover of Warhammer Visions, unless we are going to see the next two dataslates, which seems likely.
Here is a link to the image over on Warseer
and of course another location