The eveelutions should all go back to there roots: stones.
Umbreon could be Dusk/Moon stone,
Espeon could be Dawn/Sun/Shiny stone,
Leafeon could be Leaf stone,
and Glaceon could use some kind of Icy Stone or something.
As for the cornet guys, it'll probably be whatever the main mountain is in their current region, or maybe just level them up in a cave.
I think the main reason behind that is simply for the fact that in the original games, Eevee evolved into Jolteon, Vaporeon and Flareon, with the Thunder, Water and Fire stones respectively. The Moon and Leaf Stones were present in the games, but did nothing. Though it is their choice what to do with the games, I think they made new ways for Eevee (And any Pokemon, really) to evolve other then leveling up and using the stones to keep things interesting. Had they had decided to introduce those stones as the catalyst for their evolution in the later games when they debuted, people would be asking why their Eevee couldn't evolve into Espeon and Leafeon in the originals when they do 'now' (In this 'What if' scenario world). It keeps things fresh and simple when they make new evolution methods, even if a stone or something already exists that could have worked in the first place.
That being said, I think everyone had the same idea about where the Mossy/Icy rocks and what not would be placed when we were hoping for our G/S remakes. I do however like to think that some Pokemon might only evolve in a certain region, like Magneton to Magnezone and Nosepass to Probopass, much like Shellos supposedly looks different depending on what region/part of a region you're in. Whether Gamefreak decides to add in a way for most or all of these Pokemon to evolve is something we won't know until the games release. For convenience sake it would be nice if they could all evolve since not everybody wants to or can afford to buy a Pokemon game every time a new one comes out, but it would also be neat from a 'Trainer' standpoint having to trade to get those Pokemon. With the GTS, assuming they add it into HG/SS, it wouldn't be too hard to find an evolution of a fairly common group of Pokemon.