No Trainers
This place is looking like a ghost town
Imagine a world where you are the only Pokemon trainer (except for Gym Leaders and the odd Team Rocket member). Every town looks like a ghost town (even Lavender Town).
Just to spice things up a bit more - imagine if you couldn't run from ANY wild Pokemon, meaning your (likely under levelled) team is in a lot of danger. Oh, and why not make it a Nuzlocke too - just for old times sake.

here's an example...
Another hilarious run by SmallAnt - which takes an unexpected twist after the fifth Gym badge.