While I never thought of it that way, I do see two issues with thinking they mightn't do anything with older Pokemon because the regions are separate.
The first being they are a business and any good business tries to make their product make as many people as happy as possible. Many want pre/evolutions, so as a business they'd be remiss to not do so (a second time in a row at least).
The second, coming at it from a story and environmental point of view, many animals have been introduced to new areas (intentionally or not) and can and will change the environment, and be changed to suit the environment as natural selection is wont to do. So they can be easily explained as simply, for all intents and purposes, forcing a change to the Pokemon by changing it's environment.
As for me, two on my top ten list (minus babies) would be:
1.) Machoke alternate evo. Obtained by removing the Power Saver belt that locks in it's power. If something looking like Machamp results from his contained power it'd be interesting to see what an unleashed (er... unbelted) form would look like. Though it would probably end up with some sort of draw back like Slaking and Regigigas to rein in it's overall battle power.
2.) An evolution for Starmie (Starus). Not so sure about the how, but it does have that core in it. It's core somehow becoming denser and starting nuclear fusion within it making it a Water/Fire-Type and dropping some speed to gain other stats makes sense to me.
The latter actually makes more sense where I'm taking it from though. In my obsession over the years I've got information for story/Pokemon/"bad guys"/base stats/moves etc. for 3 regions. Starus is actually one of those few in my first region, but to explain how it makes sense would require recapping most of the story and make this post about 2 pages long for the abridged version and since they don't have the capabilities in the real game as in my stories - at the moment at least anyway - explaining it wouldn't even really help.
Regardless, that basis is one I'd love to see, no matter how it came about.