Maybe this already exists, but a nuzlocke where you only get one pokemon of each type, for a total of 17/18. When you encounter a pokemon, and one of its types is a type you don't have, you must catch it. If all 17/18 die, that's it.
"Ultimate 18 Challenge"
1. Feint = death, like normal
2. If you encounter a pokemon with a type you don't have, you must catch a pokemon of that species. (if you miss your chance you can try again. you MUST get one.)
3. The catching restriction only applies to what type each pokemon is at the time of captured. If it changes type later, ignore it. Charmander only counts as fire, even though he picks up a second type later.
Dual-Types are handled like this: You catch a charmander, you can never capture slugma, but if Macargo is in the wild you can catch him as your rock type.