Based on how AMD has handled GCN in the past I'd say there are two logical slots where a third Polaris SKU would slot in:
The first slot is below Polaris 11, at ~90mm², ~1.8b transistors, 512 shaders and 64 Bit GDDR5 bus. This would be a cost effective replacement of Oland and Cape Verde and a good replacement for the 15W to 25W volume SKUs on mobile. It's also a market that hasn't seen Pascal chips yet, increasing the competitiveness of Polaris.
The second slot is where Bonaire slotted in in 2012, at 160 to 180mm², ~4.5b transistors and 1500 to 1800 shaders, coupled with a 128 Bit GDDR5(x) bus. This would slot into the 55 to 75W mobile segment. The rather low bandwith per shader would be fine for a chip that doesn't see clocks beyond 1100 MHz, further pushing the mobile orientation.