If I can throw my two cents...
The new Navi 5700 cards are the best candidates for the new iMacs, while it seems AMD will introduce new more powerful ones in the range of 10 to 12 TFlops (I guess the replacements for Vega 56 and 64) by the Q4 2019.
If they did, it would get close to the new Cooper Lake Xeons (Q1 2020, but given it's Intel, I'd say at least H1) or Apple could go for using the same Cascade Lake as the Mac Pro, after all it would be awkward to sell a cheaper machine - even though an AIO - with more powerful CPUs.
The latter choice may make more marketing sense: the Mac Pro starts with an "audio studio" GPU (Radeon 580X) but then jumps immediately into the single Vega II, that can connect as a duo with another VegaII via Infinity Fabric at a later stage, therefore not directly comparable with a soldered single one.
There is quite a big gap between the two cards. It may not be only a question of investing in just one chip (Vega 20) and offering super high-end options to the monstrous Mac Pro. It could be a very territorial choice for Apple, leaving the GPU gap to the iMP. After all, nowadays the main critical factor for a purchase of a Mac, at least - allow me - among our rants here on the forum, is the GPU. Well... the iMP would fill exactly the gap between a Vega 48 and a Vega II, as it does already, with the new gen.
That would very likely mean it would still rely on the T2, instead of expecting some T3 evolution. Or it would steal the thunder of the recently released older brother.
Which, in the end, would leave the choice to Apple of whether to present a new case with the Pro or leave it to a more theatrical and widely appreciated/sold iMac later in 2020.
If you followed my cents bouncing down the stairs, my speculation at this point would be that we could see an "old case" iMP with Cascade Lake Xeon Ws, Navi 5800/5900?, T2 and perhaps BT 5.0 and HDR screens (I wouldn't count on WiFi6 until Ice Lake comes out for the new notebooks) in a September to January window.
Alternatively, I would hope for an H2 2020 release of redesigned iMacs and IMPs (maybe even together) all SSD-based with the present and upcoming Navi cards, but the rest is open to further and wilder speculation.