Very simple... Are there Pokemon that should learn a certain move, but they are unable to learn it? A move that would greatly improve that Pokemon? Mention it here!
It should at least somewhat fit the Pokemon and it should feel realistic and understandable if GameFreak would add that move to the Pokemon's movepool. So no Razor Leaf for Suicune or Surf for Celebi. ;)
Also, please do not go for moves that would just be fun, but that have no actual impact. Please go for moves that would actually really improve the Pokemon. Moves that would have made the Pokemon way better competitively or in-game. Moves that would actually make a big difference for the Pokemon you have in mind.
My first suggestion... Milotic should have been able to learn Calm Mind.
The move would fit Milotic perfectly, and it would have enabled Milotic to run Suicune's popular CroCune set (Surf, Calm Mind, Rest, Sleep Talk), but then even better than Suicune.
After all, Milotic has a higher Special Attack and Special Defense stat than Suicune. And the ability Marvel Scale gives it a huge Defense boost when it is asleep, which is why our beloved Hoenn serpent is a great RestTalker.
CroTic would have been amazing, especially in Gen III and to a lesser extent, IV. I have used both CroCune and defensive RestTalk Milotic an awful lot in Gen III and IV competitive battling and Battle Frontier, and CroTic would have been amazing. Calm Mind would have made Milotic an even better Pokemon than it already is.
I have to admit I stopped playing competitively when Gen 4 ended and Gen 5 came, so I have no idea how Suicune and Milotic are doing in the current competitive environment. This old guy is speaking from a Gen III and IV perspective, back when Suicune was amazing in Gen III and IV OU and back when Milotic was amazing in Gen III OU and one of my favourites to use in Gen IV UU.
Jumpluff should have gotten Spore. With that amazing Speed, Spore instead of Sleep Powder would have been amazing... And I think Spore fits this cutie perfectly. I do not think it would have been overpowered, since Jumpluff has poor stats everywhere aside from its amazing Speed.
It should at least somewhat fit the Pokemon and it should feel realistic and understandable if GameFreak would add that move to the Pokemon's movepool. So no Razor Leaf for Suicune or Surf for Celebi. ;)
Also, please do not go for moves that would just be fun, but that have no actual impact. Please go for moves that would actually really improve the Pokemon. Moves that would have made the Pokemon way better competitively or in-game. Moves that would actually make a big difference for the Pokemon you have in mind.

My first suggestion... Milotic should have been able to learn Calm Mind.
The move would fit Milotic perfectly, and it would have enabled Milotic to run Suicune's popular CroCune set (Surf, Calm Mind, Rest, Sleep Talk), but then even better than Suicune.
After all, Milotic has a higher Special Attack and Special Defense stat than Suicune. And the ability Marvel Scale gives it a huge Defense boost when it is asleep, which is why our beloved Hoenn serpent is a great RestTalker.
CroTic would have been amazing, especially in Gen III and to a lesser extent, IV. I have used both CroCune and defensive RestTalk Milotic an awful lot in Gen III and IV competitive battling and Battle Frontier, and CroTic would have been amazing. Calm Mind would have made Milotic an even better Pokemon than it already is.
I have to admit I stopped playing competitively when Gen 4 ended and Gen 5 came, so I have no idea how Suicune and Milotic are doing in the current competitive environment. This old guy is speaking from a Gen III and IV perspective, back when Suicune was amazing in Gen III and IV OU and back when Milotic was amazing in Gen III OU and one of my favourites to use in Gen IV UU.

Jumpluff should have gotten Spore. With that amazing Speed, Spore instead of Sleep Powder would have been amazing... And I think Spore fits this cutie perfectly. I do not think it would have been overpowered, since Jumpluff has poor stats everywhere aside from its amazing Speed.