A little extra challenge
Similar to a Monotype Nuzlocke - another challenge which can be attempted is a Monogen - i.e. only using Pokémon from a specific generation. For an additional challenge, select a generation different to the one you are playing as there will likely be less Pokémon available to you.
For example, in a playthrough of Ruby and Sapphire (Gen 3), you could select Gen 2 for an additional challenge. Available Pokémon would include: Azumarill, Magcargo, Pichu (but not Pikachu or Raichu!), Wobbufett, Xatu etc. Pokémon such as Crobat and Bellossom are unavailable as these require you to catch Gen I Pokémon and then evolving them.
You will need to tweak the traditional Nuzlocke rules when doing a monogen run:
The trainer is able to catch one of each species of Pokémon of their selected gen, rather than just the first Pokémon found on each route (e.g. the trainer can catch a Marill if they are doing a Gen 2 run, regardless of whether Marill is found first on any particular route or not). For an additional challenge, you can say that you may only catch the first Pokémon from your specific Gen on each route. Therefore if there are more than 1 available on a specific route - you might miss out on the one you want/need.
Additionally, trainers may decide to give each of their Pokémon a certain number of lives rather than using the rule of a K.O meaning permanent death. This is to counter the fact that a monogen run will generally lead to the trainer have less available Pokémon to fall back on in their P.C when compared to a regular Nuzlocke. However, this is up to you.