I'll go with SATA. maybe SATA 3.2.
I don;t think that console makers will adopt M.2 right now. m.2 drives are too expensive right now.

If you're expecting the current generation to last for a whopping nine years in total (plus overlap with the next generation), I don't know what to say. Also, I very much disagree about storage technology delaying the release of a new generation.

Well. Yes and no.
Western Digital Black 6TB is able to do almost 220MB/s in sustained transfers.

Which means it would take 1 minute, 13 seconds to fill up 16GB.

Where mechanical disks truly tank and SSD's shine however is the random reads/writes... 2MB random reads that same drive will drop to about 80MB/s... Which means it would take 3 minutes, 20 seconds.

Good thing not all transfers will be random reads.

Mechanical disks will be fine for next gen if they are fast mechanical disks... This console generation we were using very slow drives, the Xbox One/Playstation 4 launch mechanical disks were probably 1/3rd this speed at peak rates.

Plus mechanical disks will keep improving untill next gen drops.

this^

Without fast enough storage, there is no way to have a large enough pool of RAM to make a true generational leap. The earlist we could possibly see CPU/GPU/RAM/SSD all line up for a true generational leap is 2020 or 7 years. It comes down to whether they can do it in the $500 or less price range by then. As someone mentioned above though, we are going from 2 K to 4K which is a lot larger leap than 720p to 1080p. So to get a generational leap we need more power than a lot of people are talking.

I don't think there is any chane we will see a 3.5" drive in the next consoles. Also, 16GB of RAM is not even remotely next gen either, no matter how you stack it up. 

HDD's can have a nice send off with XOX and a PS4 Premium. 16GB of RAM would be perfect for a full 4K PS4. Next gen needs SSD and 64GB RAM minimum, or preferably 128GB. 

If Sony tries to label a 4 Core/8 thread Ryzen APU, with 10 -12Tflops of GPU power, 16GB of RAM, and an HDD as PS5, it will do major damage to the brand. The mass market doesn't want half step devices. Save that stuff for the hardcore, deliver true next gen machines to the main stream.

To early to tell what drives they will use, but it will be whatever is cost effective.

Once you have transferred enough data, then the rest can be streamed in.

Plus... Loading 16GB of Ram is on top of what is already loaded... Don't be surprised if 4GB+ is reserved for the OS/Background tasks, which pushes that total to 20GB... Plus the initial load-up of a few Gigabytes once you have launched the game and gotten to the game menu... And well. You get the idea.

16GB is the mid-range on PC at the moment. Hopefully by 2020 that ends up being 32GB, which is likely the upper limit of what to expect next-gen.
Consoles are at the mercy of what the PC offers. If the PC stagnates, then so do consoles.

I would hope for an octo-core CPU in 2020 at a minimum.
Ryzen will be old and outdated by then, you would be looking at Ryzen 3/4 by then on the PC.

Flops doesn't tell us how powerful a GPU is.

You want the next gen to cost 2000$???

Anything other than SATA HDDs are too expensive for a console, especially with sizes above 1 Terabyte. 1 terabyte SSD cost over 300$ while a comparable HDD is less than 100$

Worse, you are expecting 64-128 Gbyte of RAM. 64 GB RAM cost already 700$, 128 GB 1500$. And that's not even thinking about where to put all those RAM Chips on the motherboard of console, which would need to be gigantic (E-ATX size at the very least) to fit all that memory

The mass market doesn't want half step devices? Then how come Playstation 4 and Xbox ONE are selling at all? They where deemed weak from the get-go.

As for my expectations for the next gen, these are:

2TB HDD

16-24 GB GDDR6 RAM

4-8 (physical) cores Ryzen2 based CPU, 2.6-3.6 Ghz (the more cores, the less clock)

48-64 Navi-based Compute Units, 1100-1400Mhz to limit consumption

Release date: 2020

The only thing I'm not sure is if they will still use optical discs or not. Unless the price for Flash memory drops significantly they will certainly still use optical discs, otherwise the cartridges would become too expensive (as seen on the Switch already)

You mean SATA or PCIe. M.2 is an interface and some M.2 drives use flash storage that only interfaces through SATA, while others which are NVMe can use both the PCIe and the SATA interface.

Ryzen 2 is slated for 2019 (Pinnacle ridge is just a Speedbump for 2018, not Ryzen 2). Ryzen 3 may be a possibility, but there's no chance there will be a Ryzen 4 by that point already.

If you ask me they could just pop a 256 GB SSD into the next consoles and if you want more space you just get yourself an external hard drive and that's that.