Making this post for personal reference.
Platinum Mons
Kavatika, take ♥ILoveLife♥ the Chimchar.
♥ILoveLife♥'s battling strength is directly connected to how much health he has remaining. When ♥ILoveLife♥ is in battle, he can only use moves whose base power is equal or less than the amount of hit points he has remaining, not factoring in STAB. To put this in perspective, at Level 50, Infernape has 151 HP minimum, which is a little higher than his strongest moves, Flare Blitz and Close Combat.
Any items that directly heal ♥ILoveLife♥, or indirectly heal it (such as removing a burn or poison) cannot be used on it, unless you are both A: In a battle and B: It is the active Pokemon. Additionally, if you do choose to use an item on it, the move it has with the highest power will be disabled for the rest of the battle. Pokemon centers can be used normally, and moves like Wish or Heal Bell/Aromatherapy can be used with no additional consequences.
However, ♥ILoveLife♥ is very defensive of his friends. If he is the only Pokemon in the party that is not fainted, he will be motivated to save everyone, allowing him to fight without any restrictions. Additionally, Scratch may be used at any time regardless of your current HP.
If this is too hard or too easy, just tell me and I'll change it up a bit.
Good luck!
EDIT: Added the Scratch clause to avoid issues where you can't use any moves at all
EDIT: Kavatika: You get Taxing the Gible.
This Gible is so powerful... Yet so taxing... To the point that you have to bribe it to do anything. Each time it attacks, you must discard (not sell) item(s) equal to the number of Badges you have at the end of the battle. (i.e. 5 attacks, 2 badges, discard 10 at the end of the battle) Mail does not count as an item to discard. It may never be the lowest levelled Pokémon on your team. It may never leave the party.
Have fun.
Catch the first female Machop you find ASAP. This will be your mon.
Machop has a bit of a gender issue--it can't tell which gender it is. It thinks it's male, and it looks male, but it's body is that of a female. As such, it is quite confused. As a result of this confusion, it refuses to obey you when you send it out. When you battle with it, it may only use non-damaging moves for the first five turns of the battle, and if it gets Taunted it will become more insecure than it already is and will switch out right away. After the first five turns, it must use Low Kick on any male Pokemon (aim for the groin!) and Karate Chop on females (before it gets those moves it cannot fight mons of those genders). It will also refuse to fight any Genderless Pokemon. It wants to look really tough, and as such must solo Maylene to evolve into Machoke. It never evolves into Machamp because it's too cool for that.
If it is hit by the move Taunt, it is shocked by the Taunt and as such has its brain addled for quite a while. It will refuse to do anything but sit at the back of your party without holding any items or accepting any healing, for the next five trainers.
If it is hit by the move Torment, it is shocked by the Torment and becomes suicidal. Fight wild Pokemon without healing until you faint. After that, deposit her in your PC until you finish the next gym. If this happens after Volkner, then deposit her until you reach the Pokemon League, and then take her out again.
When she fights Cynthia her old rage is rekindled. Solo two of her mons.
Edit: For Platinum take Cyberspace the Porygon, Cyberspace must have Conversion or Conversion2 and may only use STAB moves (STAB move is also a move of the type you got from Conversions)
It must solo Candice with only it's original STAB moves, to prove it's the best type and Ice Type, to fit in with the flavor of the gym, still you may only use STAB moves even if you disable it in the rest of teh game
Note: If it is too hard, you may change the only use STAB moves restriction, but it must keep Conversion or Conversion2
Yellow Mons:
Take a Pikachu named Comet
This Pikachu once lived on a Comet high up in the sky, but the day you found him, he fell of the Comet and bumped his head, because of this he thinks he is the comet.
As comets only appear rarely, he has to use Double Team every time he enters the battle, if he's hit by any damaging move, he must switch out, as he has been seen, this counts at any moment during a battle, if he doesn't have Double Team yet, you skip the double team part, but still have to switch out every time you get hit.
Against any Pokemon that has a higher base Speed he can only use Quick Attack, even if he just enters the battle, he may only use Quick Attack as Comets are faster than anything and will always be the fastest, he may not battle
He may not switch out if it isn't required because of the first rule.
He must solo any Pokemon that is classified as a Pokemon from space, if you don't manage to, no excuses, you must SoftReset.
Take Infernal the Charmander. This specific Charmander believes he was born from the depths of hell, and that his fire can melt anything. Therefore, he must-by the end of the game-know only fire moves, with Infernal having to learn a fire move whenever he gets the opportunity. Infernal believes that because he is from hell, his flames cannot be put out, so solo any trainer owned water types, as well as Misty's gym. To evolve into Charmeleon, solo 10 trainer owned water types, which can include Misty's gym. Infernal also wants to be as evil as possible, so solo any Lasses as long as it does not conflict with another challange. Because of this desire to be evil, Infernal may never fight Team Rocket, including Giovanni. Infernal, being a demon, also wants to destroy the world and all its life, so solo Erika, as well as two other gym leaders, because they serve as defenders of the world. Note that Giovanni is exempt because he is the leader of Team Rocket. Finally, have Infernal solo your rival at every opportunity, because Infernal wants to show your rival who really is stronger.
Kavatika for your Yellow challenge, allow me to grant you
Reginald the Goldeen.
He's a bit of an arrogant, upper-class twit, to be honest. He doesn't expect to be the lead all the time, but for all major battles that aren't being solo'd by his teammates, he damn well expects you to use him and let him KO something. This would be slightly easier if he weren't so stupid: He has a major issue with technical machines and can't learn from them. He can only use his natural level-up pool.
Reginald is aware of the fact that in the remakes Goldeen can learn Megahorn, and thus Reginald is convinced he is the best anti-psychic weapon you have. And he expects to be used against all of them (except obviously any that are already going to be fought by Comet and Infernal). With the exception of Saffron Gym and Lorelei's Pokemon, though, he isn't necessarily required to beat them - but if he doesn't beat any given Psychic type he's going to be sulking for the rest of that area and won't do anything for you.
HeartGold mons:
Take a Mankey named MoneyMaker
MoneyMinty only cares about money, so the only move it can use is Covet, however if your money is higher than 10.000 you unlock any second move, higher than 50.000 a third move and if higher than 100.000 you can use all four moves
Take Support the Chikorita. This Chikorita wants to fulfill the role of a typical support character in most RPGs, so it needs to know a healing move, a move that causes indirect damage, and a move that buffs the party (i.e. Reflect, Light Screen) ASAP. Note that Leech Seed can be used as your indirect damage or healing move, but not both. However, support wants to show that she is strong in her own right, so solo Bugsy and Pryce, as well as any other Gym Leaders whose type is Super-Effective on Grass. Also solo Gym Leaders whose type resists Grass. If Support and an Ally are in a double battle and the ally faints, Support feels bad, so she may not be used for a half hour of real time. Finally, being the support, Chikorita must lead off before any other team member's solos to use her party-buffing move. Solo Koga as well, just for the hell of it.
Kavatika for your HeartGold challenge, take a Rattata. Name it Reiko.
This FEMALE Rattata hails from Tian Xia. Think Asia, except without dictators and such. She is particularly skilled in the way of the ninja. All her moves must consist of the following moves ASAP: Dark-type moves, Quick Attack, Focus Energy, Iron Tail, and Rock Smash. She is very adherent to a code of law. Unlike that scum Team Rocket. She must solo at least 2 Team Rocket Admins that do not conflict with other challenges. And finally, she must prove that she is the best ninja of all! She must solo Koga and Janine.
Now that I look back at my HG mons, they need to be revamped, or rejected. Rattata and Chikorita are both soloing Koga atm, and Mankey learns Covet via move relearner, so they need to be fixed else I will look for a replacement. I still need two Platinum mons, but at this point I am considering just leaving the other two unrestricted and up to me.
