I am willing to bet its Kiss or Iron Maiden. I think those are 2 wise choices that will sell well.
Now, having said there, I know there are people here just throwing out fun titles like Golden Girls etc, and those are funny, but there are also others asking for Led Zeppelin or Grateful Dead for example.
No disrespect to those posters nor bands, but they simple wouldn't sell many as a pin.
There has to be a large enough market to produce such titles. Personally I would love to see either a Pink Floyd or The Wall pin, but I don't think Stern couldn't move enough of those machines.
I am kind of caught off guard by WrestleMania. I am sure if it were still the '80s or '90s they couldn't keep that machine in stock, but I think the window on that title has closed. Now it sounds like Stern is about to take another hit on Whoa Nellie due to pricing.
Again, I am not knocking on you for enjoying that music or wrestling. I'm just being realistic about how many units can be sold and still make a profit.

This is a direct quote from Pinball magazine:

Unconfirmed rumors of upcoming titles: Iron Maiden (supposedly designed by Steve Ritchie). For whatever it’s worth: on Facebook George Gomez, Stern’s VP of Game Design, asked what people thought of Guardians of the Galaxy. At the 2014 NW Pinball & Arcade Show pinball designer John Popadiuk confirmed he was unable to secure the KISS-license after displaying his KISS-prototype at the Midwest Gaming Classics show. He had been informed the license had been secured by Stern since then. Please note that Stern have not confirmed they secured this license and we apologise if a previous listing may have given the impression they did. If true, it would most likely be a title for 2015. On the Pinside Forum former pinball designer Mark Ritchie has been briefly rumored as the designer of this game, but when asked about it at Pinball Expo 2014 Mark denied being involved. If any of these rumors are true the most logical designer to be working on it would be John Trudeau.