The new Mac mini is almost certainly coming

[sublunar]

They could have just bumped the 2014 mini to a newer Intel mobile-class chipset. Instead there was a significant shift to making it a bit more "pro", using desktop-class processors (I think people thought that Trunderbolt external GPUs were going to be a thing at the time).

As @sevoneone said - Apple also did a major U turn on desktop Macs in 2017 so who knows what internal shenanigans went on. They may have been planning to drop the Mini and go all-iMac.

At the time I recall it needing engineering resources to create a refresh mini (remember the MacBook Pro got annual upgrades so the same chipset could have been used) - bizarre thinking but then the sudden pivot to 2018 Mac mini with Coffee Lake desktop class Intel CPUs (along with thunderbolt ports) and still upgradable RAM was a pleasant surprise.

And yes I did research thunderbolt enclosures with GPUs but then found that Nvidia was off the table, AMD was limited choice and the price of adding 4x PCIe 3.0 lanes to then add a limited choice of GPUs was not a great value proposition.

YMMV but I wouldn't touch the tvOS only ATV at those prices - actually I wouldn't touch the current ATV at its new price. Virtually any modern TV has the basic streaming services (and, often, Airplay) built in and, if not, there are cheaper FireTV and Roku boxes.

Otherwise, I remain to be convinced that there is a market for - effectively - an Apple games console. I suspect that the target market will already have a games console (probably a much more expensive one) which can double as a streamer.

The "computers" App which could play material (particularly Music) from my Mac was an attraction - but with Apple pushing towards the cloud and using the Music app instead that's becoming less important, and there are alternatives like Jellyfin (...which has better clients on other platforms).

...and your last option, effectively a Mac Neo, would be interesting - but a whole other type of purchase.

The Apple TV has the biggest percentage price increase across all Apple products due to the low start price - which is already high for a streaming box. If you're not pushing the gaming aspect (admittedly it would take a couple of years to come to fruition) and TVs are starting to get the Apple TV app as well as Airplay then what exactly is it for? The homeOS powered iPad on a stick pastiche of a G4 iMac might take away the thread powered HomeKit Hub role which has already been removed from iPads.

There's already been some disquiet from the PS5 crowd about the loss of discs - Apple don't have discs to lose I guess but there are devs being released after a recent run of layoffs. The latest news seems to be Microsoft following suit with Xbox in terms of price increases so I suppose there's little reason to be surprised by the price re-alignments.

I guess on a value perspective the only thing left would be to give us a 399 USD 256Gb device that can boot into macOS as a Neo.

For me, the possible attraction of a Mac connected to my TV would be the ability to run MacOS Kodi (or JellyFin/Plex/whatever) alongside the tvOS streaming clients. So I agree that running tvOS Apps under MacOS (plus, as I mentioned earlier, a 10' interface like the old Front Row that could launch both tvOS and MacOS apps).

Well, I'd prefer long term support for macOS on ARM - if there's ways to keep it going on M1 Macs then I'm all for it.
Other than that, what else could you do with an M1 Mac mini (bringing this back on topic) in 2032 if macOS has left it behind completely? A games machine with a respectable library of games would be good, or and afterlife as a home hub - imagine if an M3 MacBook Air could perform the same job as a Home Hub with included Thread radio?

However, I'd say the concept remaining would be to try old Macs out as a tvOS based gaming platform which might actually give Apple Arcade a reason to exist.