Pichu vs. Garchomp
One of the key elements of a Pokémon playthrough is evolving your team - the game is designed for Pokémon to have large increases in their strength which allow you to beat stronger trainers in the later parts of the game.
So what if evolution wasn't allowed? For example, can you beat the Elite Four with a team consisting of Charmander, Pidgey, Nidoran, Pikachu, Staryu and Machop?
Of course - there are ways to build a strong team using non-evolved Pokémon such as Lapras, Hitmonchan and Mr. Mime. Therefore, to make this more of a challenge...
Combine a traditional Nuzlocke with these rules - i.e. you can only catch the first Pokémon on each route AND you are not allowed to evolve it. Clever use of items and strong tactics will be crucial.
As in the video below - you could try using ONLY Baby Pokémon - e.g. Riolu, Smoochum, Cleffa etc. Baby Pokémon have notoriously bad stats and therefore a lot of thought will need to be put in if you want to complete this type of run.
here's an example...
A Baby Pokémon only playthrough of Gen 4 by Pokétuber Mah-Dry-Bread.