I second this idea tbh. There was a metagame a while back that was this (I forget what it was called, Ijust know it was like an OM of the month kind of thing). Despite being very simple, it was some of the most fun I've ever had on showdown. Due to how damage works, everything at that level hit like a nuke.
I agree with
DrPumpkinz that restricting it to just Pokemon that can exist at level 1 isn't a good idea. At that stage, most Pokemon will have roughly the same stats. Sure a single stat point difference makes a huge difference, but overall there isn't much separating Chimchar from something like Rillaboom in that department. This means most Pokemon rely on their typing, move set, and abilities for strategy. Evolved Pokemon usually tend to have more options in that regard, limiting in by excluding such Pokemon I feel restricts the potential of the metagame.
In regards to bans, I think the focus should be less on Pokemon and more on abilities. While something like Technichian isn't crazy due to the need for moves with 60 or less BP, and Sheer Force isn't that crazy, but huge and pure power are nutz, the boost they give is just insane. For example, a Pokemon with base 255 attack, something that doesn't exist, would have 12 attack at its max. Marill, with its 20 base attack, also has 12 with Huge Power. Stronger Huge Power mons like Medicham and Diggersby have 14 attack. I'm happy to discuss other potential bans, but as a general rule I think abilities that give really high damage increases but with no drawback should be banned.
Edit: In that Chimchar vs Rillaboom example I randomly picked out, Chimchar would actually have better bulk due to Eviolite, so allowing Pokemon that you can't get at level 1 won't just invalidate a lot of Pre-Evos, the Eviolite boost is very useful here.