A new battleground is coming to Heroes of the Storm called Lost Cavern. This will be for custom-games only, and is a simple one-lane battleground with no real map objectives and also no Hearthing/healing back at the starting area.
It's more or less the opposite of Towers of Doom (which I really enjoy,) and uses the tile set of Tomb of the Spider Queen.
I don't know whether this is the proposed "arena mode" they had talked about earlier or if it's another idea.
I do think that adding more battlegrounds is generally always going to be welcome in Heroes - though you could find yourself in a position where there are so many that you find you aren't getting your favorites as much.
This doesn't look like a terribly hard BG for them to build, which is fine. It's also not really my style - also fine.
My hope though is that they're working on something new with the ambition of Towers of Doom. I absolutely adore the aesthetic of ToD, and the way it's constructed keeps the whole map relevant throughout the game without encouraging a stalemate.
Also, given that we're going to start getting Overwatch heroes soon, it seems high-time that we have a sci-fi themed battleground. Hell, even Warcraft has plenty of sci-fi elements. Raynor and Kerrigan have had to fight their way through medieval castles - how about making Valla and Jaina fight their way through a space station?