My Sony HB-F1XV

Since MSX was a standard licensed platform, the machine's internal hardware (CPU, VDP, Sound, RAM) were the same because they needed to all run the same software. This is aside from any expansions or peripherals the manufacturer may have chosen to bundle in the box. In general, such add-ons can usually be added via external cartridge anyway. The other thing would be retrofitting any video mods such as S-Video or component out. As with most 80s machines, that's mostly handled by modern kits you buy online, so you'll just want to check that your chosen machine is compatible with any add-on kit you want to hack into it.

Otherwise the key difference is down to which external form factor, design, colors and keyboard ergonomics you prefer. As for x68k, that's not an all-in-one so it doesn't fit my collecting profile. So, I know little about it (though I'd certainly love to have one anyway). It's a pity I was selective in my profile because I acquired my collection in the late 90s when most of these machines were free or near-free. :-) As it is, I managed to get clean, complete, working examples of every Commodore, Atari, Sinclair, Amstrad, Dragon, Thompson, Radio Shack, etc that were all-in-one - as well as most of the MSX machines and many clones and one-offs. Well over a hundred unique models and I never paid more than $20 + shipping for any of them. In fact, I haven't acquired any retro computers in the last 20 years since my collection was already complete by then. :-)

Yes, it's a little confusing because these Yamaha VDPs were a mostly backward compatible product line which evolved over time, starting with the v9918 in the TI 99/4. The MSX2+ featured the v9958 which did have 8 sprites (with rotation), tilemaps plus line, row and column scroll. https://en.wikipedia.org/wiki/Yamaha_V9958. It didn't have specific hardware for per-scanline mode switches (ala Amiga's Copper chip) but you could still accomplish such effects by old-school "baby-sitting the beam". Technically, the 6309 was specced running at up to 3.5 MHz, so we'd be overclocking it by 0.08 MHz to get it at the NTSC color burst frequency making such Copper-like timing tricks even easier to do with just the CPU, but we already know it'll easily run at that. Tangentially, this is the same reason the Amiga under-clocked an 8 MHz 68000 CPU to 7.16 MHz (2x color burst).

The confusing part is that, while the v9990 was the last, most advanced chip in the line - it ended up being artificially compromised because it was supposed to ship as the backward-compatible v9978 chip that MSX3 was going to be based on. But that chip ended up being late, so as a stopgap MSX2+ was created instead, with more RAM and an additional RISC CPU bolted on but no new VDP. Shortly after, the market moved on and MSX3 was cancelled. ASCII (who controlled the MSX platform rights) was still pissed at Yamaha for being late on the design (which they blamed for largely killing MSX's chance at survival by delaying MSX3 to death). So, when Yamaha later asked if they could offer the now-complete v9978 to other manufacturers, ASCII required them to remove some of the backward compatible aspects, including the sprites, tiles, scrolling and some bitmap stuff. So, the v9990 ended up shipping with more advanced features in some ways but in other ways nerfed by the arbitrary removal of some capabilities which were already in the base v9958 design before the v9978 improvements were even added.

As it turned out, the v9990 chip was never adopted by any computer platform and was only used in some low-volume MSX expansion cards made by small third-party add-on companies (which is what OP has in his system). Thus, the only computers actually running a v9990 are MSX2 machines which already have a v9958. So those systems have the best of both (since one can overlay on the other).

So, when we talk about the fantasy "Ultimate 8-bit", what we'd really like to spec is the v9978 which existed as finished prototypes in Yamaha engineering but never actually shipped in volume. If we want to stick with a hard "must have been a shipping part" rule, then we'd be left with speccing a v9958 + v9990 (as the OP has). The v9978 was basically just the intersection of those two feature sets, except in one chip. While no samples of the v9978 exist in the wild, the datasheet does exist: https://www.datasheets360.com/pdf/1236193767330354046

Such a 6309 + v9978 + OP4 system with 128K RAM in an Atari XEGS-like hybrid console/computer design and based on the highly-evolved MSX2+ Extended Basic ROMs (the pinnacle of MSFT's ROMs for home computers) would have been a world-beater. It could have shipped in 1988 at $149 in volume ($199 with keyboard) and easily blown away it's 8-bit console and computer competition. With clever coding, games would have looked very close to what the state-of-the-art Amiga could do and in terms of multimedia images, it's 15-bit 32,768 color stills would be even better than the Amiga's 12-bit HAM mode, yet available at less than a quarter of the Amiga 500's $800 MSRP.

Best of all it would have been a game and demo coders dream, with video and sound hardware to handle best-in-class game generation (8 sprites, tilemaps, row, line and column scroll, blitter, palette cycling, scanline interrupt mode tricks, etc), DX7-type FM synthesis and noise generation AND a "68000 Jr"-class CPU running like a bat out of hell with enough headroom to render a lot of high-value real-time objects over the tilemap and under the sprites. That's a killer combination the world never got in 8-bit platforms and didn't get in 16-bit until the Sega Mega Drive (aka Genesis). In the 8-bit era, whether computer or game console, we either got good graphics and sound hardware OR we got a powerful CPU but never both in the same machine where they could work together. It's the combination of the two that unlocked such unprecedented and surprising performance in the Amiga and, later, the Sega Genesis. And at $149 this machine could easily have been a best-seller by Xmas 1988 and had runway for a terrific 3+ year lifecycle. Even at Xmas 1992 it would still have been competitive, if not class-leader, at $99 with a mature game library.

I don't know, get out of retail hardware and potentially increase market share by allowing manufacturers to tailor devices to their respective markets? I'd just like to see more variety tbh (look at the plethora of handheld PCs), brought with a heavy dose of nostalgia for the home computer days.

The "OS and infrastructure", alongside whatever certification program to call it a "XSX", would require the use of Microsoft's XBOX game store. Microsoft would make a cut on any sales plus their own first party titles.

Without the meddling of other divisions.

Sure some of these ended up in landfills. But way, way less than 99%.

~10y after release, such old computers had use-value. Broken? Repair or use for spare parts.

As they get older, owners understand they have collector's value. And they do - see eg. eBay prices. Even a lowly ZX Spectrum easily fetches >$50 these days.

On top of that: the machine discussed never was a cheap one. Not when new, not now. Not that many MSX2+'s were built.

Never mind that most old home computers are Robust. It's usually mechanics like connectors (easy to fix), keyboards (not so easy) or power supplies (many options there).

No, the CPU is still an 8-bit Z80. It does have an upgraded graphics card in an expansion slot with a Yamaha v9990 - the next version of the v9958 graphics chip built into all MSX2+ machines. Both chips have some clever display modes which can generate "16-bit type" graphics (really 15-bit + overlay (if supported in hardware)). Those modes work by using YUV-like encoding and are similar in concept to the Amiga's HAM mode. Remarkably advanced stuff for an 80s 8-bit home computer!