What if Navi's ready this fall, and Apple knows it? Would they rather push the redesign forward, get a replacement for the butterfly keyboard out there and get some new GPUs out? The other option is to sit on the redesign for another 8 months (losing an opportunity to improve holiday-season Mac sales), simply to pick up a meh processor upgrade in addition.
My bet (and this is only a guess) is that we see a redesign in September or October, not at the iPhone event, and that it comes only in high-end configurations at first. It will replace the 15", but that will happen at the regularly scheduled update in May or so. The first models we see will be something like this:
Either one CPU option (9980 HK - today's CTO CPU) or two options with the higher one CTO, which could be either 9880H (today's standard 8-core) and 9980HK OR 9980HK and the Xeon version (assuming the Xeon can take the same RAM - Apple's not going to get into two different motherboards).
If there's only the 9980HK, they need to know that the second CPU in the 2020 lineup will be faster than the 9980HK, if only by a tiny bit. They don't want to have the high standard CPU actually go down in performance when the 2020s come out (the 9980HK equivalent will probably be CTO in 2020 as they go to the usual lineup of low, high, CTO).
New GPUs, at least as options. I'm guessing that, in keeping with the high-end nature of the initial offering, the minimum GPU should be either a Vega 16 or a Vega 20 (assuming it isn't all new options). There should be at least one option that outperforms the Vega 20 - whether it's a newer Navi GPU at the same level, a higher-end Vega GPU or one that is both higher-end and Navi.
Some bump to base configuration - will it start at 16 GB of RAM, but with a 1 TB drive, or will it start with 32 GB of RAM, but only a 512 GB drive, or will it get both bumps. The lower-end configuration that should come out in May won't get the bumped base configuration - but it'll be much cheaper.
My suspicion is that this is a high-end halo model for now, but replaces the 15" at the next regularly scheduled update. That's how the Retina was rolled out.