Hello everyone. Here I'll give a list of ways to make the games more challenging, for those of us that have completed it too many times to count anymore.
Scramble challenge- 5 people recommend you one pokemon each. Your 6th is your starter, and you use only those pokemon on your list. Found here.
Monotype challenge- You play through the game using a single type. Dual types are allowed if they have your chosen type, as are pokemon that will evolve into that type. If you can't catch one of your type before the first gym, trade or use your starter until you can- but no dragons/ghosts. Full rules and a thread here.
No running away- Self explanitory. Never run from a wild pokemon, fight every trainer.
Gym Challenger - Only take to a Gym the same number of pokemon as a Gym leader has- how is it fair when it can be 6 on 3, for example, against Gardenia?
Speedrun: Play through the game as fast as possible. This is partly luck based, partly patience based and partly good planning. (credit to BlitzBlast).
No saving before legendaries- about as self-explanitory as it gets.
No Evolution: You aren't allowed to evolve your pokemon. (credit to ayeepryll)
Paul style challenge - Only use pokemon with a type disadvantage against the gym leaders (eg, only Staravia and Chimchar against Roark, etc) credit to foxyman1167.
Limited items (or "we're in a recession, so we need to save teh cash!" mode as my friend dubbed it) - don't use any item that isn't required to finish the game (items like the HM surf)- includes Pokeballs, healing items, TM's etc - use none of them! Obviously, it may be necassary to use a pokeball or two on HM slaves. This mode can also be altered to be only not using potions, or not using hold items for example.
Single pokemon mode - Get your chosen pokemon ASAP and never use anything else. Or, if your chosen pokemon can't be caught early in the game, trade for it; ths might actually be harder, as if you overtrain early on it won't obey you (thanks to MKFC for pointing that out). I've seen videos of a guy who did this with a weedle on red- and won.
Hardcore mode- Any pokemon that faints gets released. Death is permanant!
Legendary- play through game without saving. (Means no using Underground and no using PCs. Until the end of the game where it saves after you beat the Elite Four.) (credit to [Super-Fjord]
For scramble and monotype mode, post on the relevant thread. If you want to post your progress on any other modes, feel free to post here. if you have any other suggestions, feel free to post too and I'll probably add them to this post.