The problem with an MBP update is that there's nothing to update them with (other than a redesign). The processors and GPUs are current and the best available from Intel and AMD, the RAM (finally) and SSD options are close to as good as it gets in a thin-and-light workstation (there are a few with very expensive 64 GB options). There's no workstation-class thin-and-light with graphics that are any faster than the Vega 20, especially at non-game tasks.
Most faster laptop GPUs are found in much larger, gaming-focused laptops. There are a very few relatively thin and light gaming laptops that offer NVidia RTX 2070 or 2080 GPUs, but at huge costs in battery life. Razer isn't even quoting battery life on their higher-end Blade models - but the lower-end model with a GTX 1060 is listed as "up to" 6 hours, which will be much less if you run Lightroom or anything else that loads the system. My suspicion is that, when they finally release the figures, the 2070 and 2080 models will be 3-4 hours browsing the web and measured in minutes for anything that really uses the GPU?
CPU rumors vary between Yet Another Skylake Derivative a few hundred MHz faster with the same core counts around the summer on the one hand and Sunny Cove on a 10nm process with substantial architectural improvements at the end of the year on the other. There is also
at least one article that seems to imply a final generation of 14nm CPUs (at least for Intel NUC mini-desktops) with a core count bump - although the availability is late enough (early 2020 for the 45w chips) that they might be talking about limited-availability Sunny Cove chips.
Intel occasionally puts chips in NUC boxes that they can't yet make enough of for more mainstream machines. I wouldn't find it at all hard to believe that they might release a Sunny Cove NUC in, say, February 2020 using a chip that wouldn't be available in Apple quantities until summer. The same chip could technically be "shipping" in a Razer Blade or the like in March or so, even though Apple might not see them for quite a while - Apple uses many more 45w chips than small vendors like Razer.
GPUs are probably in the same time frame - but Apple just did a GPU bump without calling the MBP a new model. I'd be very surprised to see them do two successive GPU-only (or even essentially GPU-only) upgrades.
Apple might or might not bother with Yet Another Skylake Derivative (assuming that it doesn't bump available core counts). Right before Skylake, they ignored Broadwell chips perfectly suitable for a minor update to the MBP, keeping Haswell on the market from late 2013 until the emergence of the Touch Bar/Skylake generation. They would be crazy to ignore something with a core count bump, and they
won't ignore Sunny Cove when it comes out.
What would they do at a redesign? Screens with higher resolutions and smaller/nonexistent bezels seem logical. Could they go to 5K on the 15", which might become a 16" by killing the bezel? They'll give the keyboard another try - but it'll still be very flat - they use essentially the same keyboard across the line, and it has to fit in the MacBook. It'll either be another generation of butterfly or (if the redesign isn't for a few years) perhaps something more exotic that uses haptics. Sunny Cove makes it easy for them to support 64 GB RAM options if they want to (or 32 GB but going back to low-power RAM). They're already offering up to 4 TB SSDs (at a price), and there's nowhere else to go right now. AMD is working on attractive 7 nm GPUs.
What about alternative chips? AMD makes nothing suitable for a 15" MBP, and has shown little interest in that small market. Their highest-end mobile chips right now are quad core with relatively low base clocks, while Intel offers six cores at a faster clock speed. Of course the AMD chip is much less power hungry (35 watts including graphics, while the Intel chips are 45
plus graphics). A 45 watt plus graphics Ryzen could be very interesting, as could a ~ 85 watt chip that includes the Vega 20 (allowing better balancing of CPU and GPU loads - don't need the GPU for a given task, let the 8-core CPU pull 70 watts). Unfortunately, there is no mobile Ryzen anywhere near that class...
Apple's own CPUs are a long way from the MacBook Pro. There are two problems - one is that the MBP, especially the 15", needs a faster, more power-hungry core than anything in the A-series right now. A big core is a huge commitment, while there are a lot of Macs that could get by with the same cores as the iPad Pro - perhaps more of them. The second is that the MBP market needs really broad software compatibility (including Boot Camp and Parallels). The logical first step with A-Series Macs is restricting them to the Mac App Store, where Apple can make sure to hand out the right binaries.
My speculation is that the A-series will appear first in the MacBook, and will spread to the Mac Mini and the lower end of the iMac line. This is a guess, but what if Apple uses the designation "Pro" to identify Intel Macs? Could this be what we see by 2022?
MacBook = extreme thin and light, A-series (an iPad Pro chip will do), Mac App Store only, might come in two screen sizes. Higher-end models might increase core count above the iPad Pro.
Mac Mini = media consumption oriented (somewhere in between a Mini and an Apple TV), A-series (again, really an iPad Pro chip), Mac App Store only.
iMac = 21" and a larger screen size (could be a living room size larger than 27"), A-series (new chip using more of the iPad Pro's fast cores - think of it as a double iPad Pro chip). No chin or bezels, since the A-series chip is easier to cool. Mac App Store only
And the Intel lineup:
MacBook Pro = thin and light Intel workstation in 13" and 15" variants (maybe 14" and 16" if Apple goes bezel-free). 15" uses the fastest mobile chips available and discrete AMD graphics. Software from anywhere (including Boot Camp and Parallels).
iMac Pro = (note that the 27" iMac becomes an iMac Pro, while the 27" iMac Pro gets a 32" 8K screen to go with its Xeon). Fast, powerful Intel all-in-one. 27" and 32" versions. 27" uses fast desktop CPUs and midrange GPUs, while 32" uses Xeons and AMD's best GPUs. Needs chins and bezels to cool. Software from anywhere.
Mac Pro = High-end Xeon workstation. Software from anywhere.
Mac Mini Pro???? = Similar to 2018 Mini. Uses top-end laptop chips. Software from anywhere.
All of the "Intel" Macs could be Intel or AMD - the only problem with a Ryzen-Threadripper lineup is the 15" MBP. The 13" MBP would work with AMD's highest-end mobile chips, and the desktops all work (Ryzen for the 27" iMac that gets a "Pro" designation and for the "Mini Pro", Threadripper for the 32" heir to the current iMac Pro, Threadripper or Epyc for the Mac Pro).
The Mac Mini Pro is confusing. Apple needs to offer something Intel (or AMD) on the desktop below the 27" iMac that becomes "Pro". They could either have a 21" "iMac Pro", which seems like a real stretch, offer lower-end configurations of the Mac Pro, or do a Mac Mini Pro. I think the Mini Pro is most likely, because it continues to avoid a gaming-oriented "slotbox" that Apple has been carefully avoiding for many years. They want to keep any expandability very high in the line, where it benefits Hollywood and the high end of the photo and music markets, not gamers looking to stick a GeForce in their Mac - simply because they don't want the support hassles that odd gaming configurations and oddly behaved games bring.