While Fiji will have the enhancements that Tonga brought (better testallation performance, memory compression, etc) most rumors point towards it being called GCN 1.3 and that we should expect better performance per watt. But this is all speculation.We do know that. Fiji is based on GCN 1.2, just like Tonga. That means that, just like Tonga, it will have DX12_0 Tier 3 support but no DX12_1.
We have no idea how different it will be compared to GCN1.2. We'll just have to wait for more details at E3. I'm not an expert, but a refresh of an ahitecture can add new features. Hope dies last!