Some of my ideas for this game:
Keys: Key can be found after defeating an enemy or just lying on the ground, when it is brought back to the ship it can be used to unlock a door, they should appear mainly in caves but some could be on the overworld too.
4 player mode: Up to four people can play as Olimar, Louie, The President and Olimar's son in 3 game modes, capture the flag, duel (players battle until one captain is standing), and collect he marbles (just like the vs mode from pikmin 2)
4 playable characters: Olimar, Louie, The President and Olimar's son are all playable, they are unlocked at different points in the game, they all have their own advantages and disadvantages.
New pikmin species:
<div class='spoiler_toggle'>Spoiler: click to toggle</div><div class="spoiler" style="display:none;">Remember at the end of pikmin 1 where Olimar saw all the other coloured pikmin onions in the sky?</div> Two to three new pikmin types should be added: Green, black and possibly pink.
Green pikmin can't be thrown as far because they are thorny and hard to hold, but they are much better fighters than the others, they also stick to enemies and can't be shaken off as easily.They can see enemies from a much further distance than other pikmin so need to be kept a close eye on when there are a lot of hazards about.
Black pikmin can't be killed from being squashed, they just get replanted, however they are so stupid they need to be issued a task before they do it, otherwise they just stand there, this means they won't attack an enemy that comes close to them.
Pink pikmin are weak, they can't fight very well and run away from anything that moves, however they are very quick at carrying anything and due to their cowardly nature, they won't run in the path of an enemy while carrying treasure, they can be used to find enemies who hide in places you can't see them since they will cry and run in the opposite direction when they see one.