Apple is (wisely, in my opinion) trying to move all spinning rust outside the case. There is really no reason to have a hard drive as an internal component - it's physically large, mechanically unreliable, and loses nothing by being external. USB 3.0 has enough bandwidth for any single-drive and most dual-drive configurations, while Thunderbolt 3 can handle anything except a VERY large and specialized RAID. Something like 8 drives in RAID 0, 16-24 drives in RAID 5 or 6 or an 8 drive unit with heavy SSD caching might begin to stretch Thunderbolt 3 (it might use an x16 PCI-e card and a specialized interface cable). I would strongly expect the new Mini to be flash-only, and I'm as close to 100% sure as speculation can ever be that the modular Mac Pro will be flash-only. There will certainly be plenty of ways to hook up spinning rust outside the case.
There are 3 real possibilities for the new Mini, at least technically (Apple won't build some of them because they're Apple).
1.) The first and most likely has nothing to do with Mac Pros - it is essentially a headless laptop. The Mini has always been a headless laptop, and "we're focusing on pros" could mean nothing more than "it's a headless 15" (or even high-end 13")" laptop instead of "it's a headless low-end 13" laptop". It would have either integrated graphics only or perhaps low-end discrete graphics (think MacBook Pro level) if the chips they use had inadequate integrated graphics. It might (or might not) have a few more ports than a MacBook Pro - a couple of standard USB ports, HDMI and/or a (10G?) Ethernet port seem like decent possibilities. If Apple's feeling especially generous, it could have user-expandable RAM.
2.) The second possibility makes a lot of sense, but I think it's unlikely since Apple is Apple. It would be an iMac (if we're lucky, a 27" iMac) without the screen. It would be a great desktop, oriented towards pros who don't need the 5K monitor (either because they're in the photo/video/graphics business and use high-end Eizo and NEC monitors OR because they're musicians or software developers for whom the iMac monitor is overkill).
Despite years of forum clamor for this type of machine, Apple has assiduously protected the iMac since the day it was released 20 years ago. The last time we saw a desktop that neither underperformed the lowest-end iMac nor was more expensive than the highest end iMac was the beige PowerMac G3 (the Bondi Blue G3 was already more expensive than an iMac)! They've allowed headless laptops that are both cheaper and slower than any iMac, and they've allowed Mac Pros that are faster, but more expensive - but the iMac's price and performance range (which has moved upwards through the years) has always featured nothing but iMacs. The closest Apple ever came is that the highest end (mobile i7) version of the Late 2012 Mini performed almost identically to the base (desktop i5) version of the 21.5" iMac.
If they were to break this rule now, they'd release a great little pro desktop. Don't get your hopes up about internal expansion, although it might well have socketed RAM and there's some chance of a replaceable SSD. It would only have a ~200-250 watt power budget, so the 95W desktop CPU would go with a non-replaceable mobile Radeon. Plenty of Thunderbolt, several USB ports, and 10G Ethernet round out the package. If you don't need much GPU, it's ready for you with maximum CPU up to an 8700K or the October 1 9700K or 9900K (the 27" iMac would have to be updated to the same CPUs alongside it). If you do need GPU power, it's still worth considering with an eGPU. Unfortunately, you're dreaming - Apple has spent 20 years avoiding this thing!
3.) There is a slight (much less than a laptop derivative, but greater than the mainstream desktop they hate) chance that we'll see an iMac Pro without the screen. We don't know for sure that the iMac Pro is a protected model in the way that other iMacs are. If we see a fast, but limited expandability iMac Pro derivative, it is a direct signal about the Mac Pro. The Mac Pro will then be extremely high end - they will not release three versions of similar Xeon-W machines. If we get this one with a built-in Vega (plenty of ports for external expansion, but internal expansion limited to RAM and maybe SSD), the Mac Pro is certainly going to be Xeon-SP, may be late 2019 and may start above $10,000.
If we see only a laptop derivative Mini, it tells us nothing about the modular Mac Pro.
There are 3 real possibilities for the new Mini, at least technically (Apple won't build some of them because they're Apple).
1.) The first and most likely has nothing to do with Mac Pros - it is essentially a headless laptop. The Mini has always been a headless laptop, and "we're focusing on pros" could mean nothing more than "it's a headless 15" (or even high-end 13")" laptop instead of "it's a headless low-end 13" laptop". It would have either integrated graphics only or perhaps low-end discrete graphics (think MacBook Pro level) if the chips they use had inadequate integrated graphics. It might (or might not) have a few more ports than a MacBook Pro - a couple of standard USB ports, HDMI and/or a (10G?) Ethernet port seem like decent possibilities. If Apple's feeling especially generous, it could have user-expandable RAM.
2.) The second possibility makes a lot of sense, but I think it's unlikely since Apple is Apple. It would be an iMac (if we're lucky, a 27" iMac) without the screen. It would be a great desktop, oriented towards pros who don't need the 5K monitor (either because they're in the photo/video/graphics business and use high-end Eizo and NEC monitors OR because they're musicians or software developers for whom the iMac monitor is overkill).
Despite years of forum clamor for this type of machine, Apple has assiduously protected the iMac since the day it was released 20 years ago. The last time we saw a desktop that neither underperformed the lowest-end iMac nor was more expensive than the highest end iMac was the beige PowerMac G3 (the Bondi Blue G3 was already more expensive than an iMac)! They've allowed headless laptops that are both cheaper and slower than any iMac, and they've allowed Mac Pros that are faster, but more expensive - but the iMac's price and performance range (which has moved upwards through the years) has always featured nothing but iMacs. The closest Apple ever came is that the highest end (mobile i7) version of the Late 2012 Mini performed almost identically to the base (desktop i5) version of the 21.5" iMac.
If they were to break this rule now, they'd release a great little pro desktop. Don't get your hopes up about internal expansion, although it might well have socketed RAM and there's some chance of a replaceable SSD. It would only have a ~200-250 watt power budget, so the 95W desktop CPU would go with a non-replaceable mobile Radeon. Plenty of Thunderbolt, several USB ports, and 10G Ethernet round out the package. If you don't need much GPU, it's ready for you with maximum CPU up to an 8700K or the October 1 9700K or 9900K (the 27" iMac would have to be updated to the same CPUs alongside it). If you do need GPU power, it's still worth considering with an eGPU. Unfortunately, you're dreaming - Apple has spent 20 years avoiding this thing!
3.) There is a slight (much less than a laptop derivative, but greater than the mainstream desktop they hate) chance that we'll see an iMac Pro without the screen. We don't know for sure that the iMac Pro is a protected model in the way that other iMacs are. If we see a fast, but limited expandability iMac Pro derivative, it is a direct signal about the Mac Pro. The Mac Pro will then be extremely high end - they will not release three versions of similar Xeon-W machines. If we get this one with a built-in Vega (plenty of ports for external expansion, but internal expansion limited to RAM and maybe SSD), the Mac Pro is certainly going to be Xeon-SP, may be late 2019 and may start above $10,000.
If we see only a laptop derivative Mini, it tells us nothing about the modular Mac Pro.