Eveelutions were regional varieties before regional varieties existed. I'm sorry to tell you this, but you and your friend only came up with the same concept Gamefreak did in the nineties and which they later extended to other species, in a much more narrow way.
You only have to look at them; they're all the same base archetype, with slight shape modifications according to their typing. That's the very concept of regional varieties. Of course, you might say that "regional" is "specific of a region", but that's only true regarding distribution (that is, a purely mechanic aspect, not conceptual), so gamefreak can have an out when they leave the concept aside come the next generation; regional varieties are about specific environments more than about the region, and the best two examples are Alolan ninetales and Sandslash; a (fire type, to boot) pokemon that lives in forests and one that lives in the desert "adapt" to a new, icy environment in the same way Eevee does to become Glaceon, with the difference that they are stable enough to do it as separate species rather than as a result of impromptu, direct evolution (which is, if anything, more biologically plausible, even for pokemon).
That's it, inside the game and outside of it. Eeveelutions are a machine to make money, but even gamefreak might have a limit and work with the ton they already have rather than make new ones (they're certainly not hurting for material for merchandising). Of course, they have made new ones in the past, so we can never rule that out, but in any case, that's a moot point; theorizing about eeveelutions has been done to death, because they're easy. Because, as I said, they're all the same archetype, an easy one, any fan can mix and match to a typing of their choosing and a concept of their preference (if they even muster the effort to go that far); if any, the prevalence of eeveelution-theorymon says more about the fans than about gamefreak, because, no, gamefreak won't "run out of creativity" spending it all in eeveelutions, that's just a dumb argument; at most they'll run out of design time and pokedex space that could be used in new, non-eevee-related pokemon, but they have, and always had, creativity to spare, even for the eeveelutions.
The reality is that the fans' desire for eeveelutions to continue proves that the fans are happy with the same regurgitated idea over and over* (not to mention the sheer amount of obsessiveness about "completing the set", which they by definition view as incomplete, but I suppose that's to be expected in pokemon, of all things); it proves they're the ones who would prefer the familiarity of a four-legged fox-dog-like creature of a specific type with a somewhat constrained variety of attacks before facing a situation where they don't exist, and the "set" is incomplete, like a full binder with an empty spot, for a card they were never able to find or buy. That's not healthy.
And sure, regional/environment-dependent species are a great idea (just, not new, as I said)... when they're applied to different pokemon, and not just the same over and over; that's why eeveelutions are not worth it to keep making them and regional forms are, not because they'll be uncreative, but because the design space is too narrow, so creativity just doesn't play as much as it could; gamefreak always does the best it can, my favorite pokemon designer made many of the eeveelutions and she loves them, but speaking in terms of the design diversity, it's just more of the same, with a new type. Same reason with the pikaclones; at least the starters have a lot of room to grow and diversify, but eeveelutions are restricted in type, attacks, overall shape and pretty much all that is to be designed is the aesthetic representation of the type itself when applied to that basic template, which, while it's no small feat, it is a boring one when it becomes "just one of many", "just to complete the set", at least in comparison with the infinite potential of doing anything else that isn't an eeveelution; even pikaclones have more variety, even if they're just rodents; they have mimikyu! that's just impressive, that they grabbed that boring, repetitive model and came up with something so outré... but it's not something you can do with eeveelutions; they might be the most constrained of all pokemon archetypes.
So the short answer is that regional eeveelutions are just regular eeveelutions that you'll get only somewhere specific. So, a worse eeveelution; at least as they're now, they're relatively easy to get.
By your idea, I could simply take my vaporeon, jolteon, flareon et al and change their names to "Eeveeon", telling you they're all regional (that is, sub-regional; one's in a lake, other in a mountain, other in a volcano, etc.) variants of the same pokemon that evolves from eevee; that's your idea, just extended to the very origin of the eeveelutions; it doesn't change anything, just the names; they're still eeveelutions and they're still constraining, design and diversity-wise. In fact, if we go into the weeds, they'd be even more constraining than usual, since you wouldn't go from the basic eeveelution template, but from the specific template of the specific eeveelution you're changing type.
That doesn't mean there won't be new eeveelutions, of course, but it does mean there's not much of an argument in favor of them, in any shape, other than "one of the designers had a really good idea to add this one" or "we'll make mountains of money adding these two and reviving the entire bloody line for merchandising". Sorry again, but you'll note that those arguments all rest on gamefreak's decision, rather than the fans' need to complete the set. I think they should let it go.
* something one can also connect to the dismal evolution of the games over time, in relation to how the industry standards have changed; we still don't have multiple save slots, and didn't get player customization until the last generation (and even then, it's... hardly impressive, considering other games); most importantly, the plots are narratively dead, which makes a strong statement that gamefreak will dumb down everything they need if the fans accept it, instead of playing to their strengths and strengthening their weaknesses (they might learn a thing or two of the philosophy of the competitive players).
