There's a rather lengthy wish list features thread already here...
https://forums.macrumors.com/threads/1683404/
As for what's realistic...
- Haswell CPU (entry level 6-core?)
- DDR4 RAM
- New GPUs
You're probably looking at a 6-8% CPU performance boost clock-per-clock. I wonder if Intel will offer a low priced 6-core around the $350 mark that would allow Apple to offer an entry level 6-core for $2999.
As for I/O:
SSD: I use to think that a second SSD might be a no-brainer in a refresh, but someone pointed out that there may be a limitation on the pin count of the GPU to Motherboard connectors. Given the GPU without the SSD currently has to offer connectivity for 6 displays, I'm not sure there are enough pins to offer another x4 PCIe connection on that interface.
USB 3.1: Highly unlikely to see this in the near future in any Intel chipset and thus even less likely in any Apple product.
TB3: This is a possibility with support for PCIe 3.0 over Thunderbolt, but I'm skeptical of it being ready in 12 months.
HDMI 2.0: Very likely as this will likely be natively supported by the next gen GPUs.