I recently had an idea for a challenge about specific type combinations. Sadly, I don't have the time to participate in it, so I'm posting the idea here for anyone who might want to make an official topic about it. Also, any comments would be nice, too :)
Dual Type Challenge! [Gen II and later]
Rules:
1) When signing up, you must choose one of these predefined type combinations, and you must follow the restrictions for your chosen pair:
Hell: As hell is demonic and fiery, the Hell types are Dark & Fire. Restriction: Since Hell is about hurting others, not helping, you may only heal at Pokecenters. That means you can't use any healing moves or potions.
Purgatory: As purgatory is, more or less, spirits on "Earth", the Purgatory types are Ghost & Grass. Restriction: Since purgatory is basically a place where souls are trapped, unable to escape, you must also be trapped. That means you cannot run from any wild encounters, and you must battle every trainer--no skipping!
Heaven: As heaven is paradisiac and "in the sky", the Heaven types are Fairy & Flying. Restriction: Heaven is about goodness and not harming, so you cannot use any One-Hit KO moves. In addition, if you accidentally OHKO an opponent in a trainer battle, the Pokemon that did this must be boxed until you get your next gym badge, as punishment.
Temple: A good monk knows he must balance both mind and body; as such, the Temple types are Psychic & Fighting. Restriction: A temple is a place of silent worship. That means you cannot use any sound-based moves. In addition, whenever your Pokemon is hit with a sound-based move, you must immediately switch out to a different Pokemon, and it can't be used for the rest of the battle. The only exception is if every Pokemon in your party is hit with a sound move in the same battle, then you can go back to using whoever you want until the next sound move hits. Bulbapedia has a list of 24 moves considered "sound moves"--that's the list you're using here.
Dojo: Martial arts is about balancing strength and grace, so the Dojo types are Steel & Water. Restriction: Martial arts is about self-defense, not hitting first. Therefore, your Pokemon may not use a damaging move until the opponent damages them first. They may use non-damaging moves, or you may use an item, or you may switch Pokemon, but you cannot damage an opponent who hasn't damaged that Pokemon first.
Summit: A mountain can be snowy at its peak, or a volcano in disguise, so the Summit types are Dragon & Ice (dragon=fire breathing...I know, it's a stretch). Restriction: At the top of a mountain, there's no salvation for temperature-based afflictions. That means you cannot heal a burned or frozen status ailment without a Pokecenter.
Archeology Dig: Many extinct species were giant bugs, but now all that remains are calcified fossils... Rock & Bug are the Archaeology Dig types. Restriction: Fossils are old. Like, very old. So your Pokemon may not use any moves that were introduced in a generation after that Pokemon first appeared. For example, you couldn't use Aerial Ace with a Pidgey, since Pidgey is Gen I and Aerial Ace is Gen III. You could, however, use Aerial Ace with Beautifly, since he came in Gen III as well.
Biomimetics Lab: Biomimetics is the use of technology to mimic natural organisms; as a result, such a lab combines both technology and nature. The types are Electric & Ground. Restrictions: Since biomimetics is all about imitation, in order to complete this challenge, you must use at least one of the following moves against the final battle of your game: Transform, Mirror Move, or Copycat. If you beat the game without using any of those moves against the final battle, you have failed the challenge.
2) The only Pokemon you may catch are normal-type Pokemon and those with at least one type in your chosen pair. For example, if you chose Heaven, you can catch a Clefairy, a Pidgey, or a Togepi, but you cannot catch a Natu (because of its Psychic typing).
2a) If playing in a generation before the existence of the Fairy type, Pokemon which attained the Fairy type later can only be caught if you're playing with Heaven chosen, and only if all the rest of its types match the Heaven pair as well (or are Normal).
3) Gift and static Pokemon may be in your party even if they don't match your typings, but they can only be used for grinding and never against a trainer. If you're allowed to choose from a set of gift Pokemon, including starters, and one matches your types, you must choose that one.
4) If you encounter a Pokemon that has both of your paired types, you must attempt to catch it. You cannot flee, and you cannot knowingly kill it. In addition, if you accidentally kill such a Pokemon, you must choose one Pokemon from your party and release it (or permanently box it, as per Nuzlocke standards). This is as a punishment for killing the embodiment of your types. (Note: most of the pairs above have very few evolutionary lines with both types. Just saying.)
5) If you have a Pokemon in your party with both typings, it must lead your party. If you have multiple of these, you may choose one to lead.
6) OPTIONAL: You may play with a randomized ROM if you think it'll help you encounter the correct types better/earlier, but if you do, you may only randomize the wild encounters, not anything else. (Randomizers typically have the options to specify this.)
Goal: To beat the game as fully as possible following the restrictions above. A super-goal would be to repeat this for each type pair.
Notes: You may also note that Poison is not one of the types. That's because it would have been left with a Poison/Normal pair, and that would make Normal unobtainable in the other pairs. Which, since it's literally normal, wouldn't be thematic (and plus, using Normal as a wildcard helps expand every pair's possible Pokemon). Sorry to the Poison lovers out there.
*EDIT* Added gameplay restrictions for each type pair, to add a layer of complexity to the challenge, as well as a bit of theme.