It's pretty simple actually. If we're (really) lucky we get some form of Vega as a BTO for the high end iMac (Please note, I'm NOT talking about the high end Vega part which is said to destroy the 1080/Pascel Titan X), with Polaris based chips on the 470-480 range filling out the standard configurations.
If we're less lucky we get an overclocked binned version of the current 480 as a BTO (a 485/490 or some such).
If Apple continues on a path of mediocrity we get 460 on the base SKU, 470 on the high end SKU, and a bog standard (or even down clocked) 480 as a BTO option.
The only solace in the iMac not being released this month is that it would tend to suggest they are waiting for SOMETHING, and I sure to hell hope it's not just Kaby Lake (I really don't know why everyone is so hung up about 6700K vs 7700K, its the GPU you should be worried about people not a ~5-10% increase in CPU performance).