TBH, they don't actually need to limit variation to just one region. Why can't we catch ground type Sandshrew in the desert and ice/steel Sandshrew in a snowy area, but still within the same region?
If I use Noctowl in Johto as an example, Noctowl that are evolved from Hoothoot caught on grassy roads like Route 29, 30, 31 etc could be the standard Normal/Flying types, Noctowl caught in the Ilex Forest or the routes around Ecruteak City could be Ghost/Flying, more resembling the Rufous Owl of Australia, and Noctowl caught near Ice Path could be Ice/Flying, more resembling the Snowy Owl
As for other variant ideas...
1) How about an electric/fighting variant of Raichu, with an emphasis on kicking moves. I mean, look at Raichu's feet and tell me that thing shoundn't be using Jump Kick, Hi Jump Kick, Blaze Kick, Mega Kick etc.
2) A water/steel variants of Krabby and Kingler. Make and area where mining for iron ore has taken place and as a result Krabby living in a nearby river system have developed the steel typing from accidentally consuming metallic substances that have been washed into the water way every time it rains.
3) Fairy/Flying variants of Hoppip, Skiploom and Jumpluff.
4) Poison/Ground variants of Skorupi and Drapion
5) Dragon/Water variants of Gible, Gabite and Garchomp - make them a bit more fish-like, losing claws from their limbs and making the fins dominant. Lore could be they are a relic population of how Gible, Gabite and Garchomp originally were before developing the ability to walk on land.
6) Water/Poison variants of Mantyke and Mantine - in terms of designn basically merge them with the Bluespotted Ribbontail Ray, especially in terms of colour and adding a stringer on the tail. These pokemon live in the same waterways as the dragon/water variants of Gible, Gabite and Garchomp, so they developed the poison typing in order to deter these predators.
7) Water/Poison variant of Octillery - for the same reasons as the Mantyke/Mantine change
8) Rock variant of Castform... ok, not exactly in the same vein here, but it's long past time this thing got a Sandstorm form!
9) Fighting/Fire variant of Granbull - I'm not sure why, I just wanna see it!
10) Fighting/Flying variant of Staraptor