There's mechanics that have improved over the years, graphics, Pokémon availability, general game design, etc. Not to mention, there's also the experience of seeing an old region upgraded to modern standards.
Both of those things would be pretty challenging with Kanto and would require extensive reworking of the region. I'd be cool with that RBY/FRLG/SG/HGSS Kanto is, as I said, boring, bland and barren.
1) With the Pokemon availability we'd have to include more mons than the 150 that originally populated Kanto. By modern standards the Kanto dex is atrocious, especially when it comes to type availability. There's some Fairy types and two lines of Steel types, but no Dark types at all, the only ghost line is part Poison and there's also only one Dragon line. The only Ice Pokemon that aren't part water are a legendary and Nicky Menaj (available only through trade)
2)Not just the graphics, but level design was still much more primitive in Red/Blue. The layouts of routes and cities, the re-use of the same building sets in every city.... To get places as varied, beautiful and spectacular as, let's say, Poni Canyon, Konikoni Town, Reflection Cave and even Skyarrow bridge, would require extensive redesign.
It's not as simple to as just transliterating the existing maps to 3D environments, Kanto would have to be given a completely new identity, face and flavour in order not to seem bland in comparison to the newer regions. This would possibly entail expanding both existing locations and adding new ones.
Then there's there's the question about whether anything should be done about the design "quirks" Kanto had; should the gigantic underground dungeons with their millions of Zubats swarming you and the murderous water routes with their hordes of Tentacools be reworked to be less ridiculous (I think so) Oh and there's also no way I'm going back to HM slaves, so something needs to be changed there as well...
Especially 2) would be beyond simply adding the region as a post-game feature, even if one ignores the data constraints and the fact that a complete story would have to be added to, again, not make Kanto bland and lifeless in comparison to Alola. So it would be more the province of (yet another) remake rather than a bonus feature.
And considering that the last remake did absolutely nothing to improve upon Kanto during the storyline itself (except the graphical update) and even went out of the way to prevent you from getting any Pokemon beyond the arbitrary 151 Kanto mons (no your Golbat can't evolve right now, even though it's nearly exploding with love for you, because you don't have the Pokedex upgrade) I'm not looking forward to another Kanto remake if it ends up like FR/LG.