The keyboard, the screen and the redesign almost have to come together, whenever they come...
Yes, it would be possible to make minimal changes to get a keyboard 1mm or so thicker into the existing MBP case (they'd have to deepen the keyboard well, which might have knock-on effects on everything below it - or they could deepen the well while also making the whole lower case 1 or 2 mm thicker, to avoid the effects on things below). It would also be possible to get a slightly larger screen in by modifying only the top case, by reducing the bezels - but you can't get all the way to 16.4" that way. Remember that Dell's InfinityEdge screens (about as close to truly bezel-free as laptops get) actually have about half the bezel of the present MBP.
If we were looking at one of these two changes (especially just squeezing in a keyboard, but also if the screen was growing a tiny bit), and it were mid-cycle, I'd say "sure, that could be a mid-cycle update - not really a new design". First of all, we're past year three of a generally four-year cycle. Second, we are hearing about a specific screen size that won't fit by simply massaging the top case. If we're looking at a 16.4" screen, that's a notable stretch to the outside dimensions.
If Apple followed expected cycles, a redesign would hit in spring/summer 2020 - but pushing it forward could make sense... The processor update for 2020 (in the big MBP - little one could get Ice Lake with a huge graphics bump) is going to be underwhelming. Unless Intel squeezes 10 or 12 cores into a mobile part, we're looking at 14nm+++++++, YetAnotherLake, 8 cores, probably with a +100 MHz base clock speed and maybe +200MHz single-core turbo. It doesn't matter, in other words.
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.