1. i think they should keep the original Pikmin and give them new abilities, like yellow pikmin can stun enemies and activate electrical switches, red pikmin can set stuff on fire, and blue pikmin can....can...ill get back to you on that.
2. scrap white and purple pikmin in favor of one or two new pikmin types like green(that can camouflage themselves and sneak by enemies or something) and maybe fuzzy pikmin or something that can withstand cold temperatues and carry alot like the purple ones could.
3.more dynamic and challenging puzzles that really require you to think about your team and use you rescources more wisely.
4. greater variety in the regions, and regions need to more open, allowing for several solutions to a problem like getting to a new area or finding a new whatever it is you'll need to find.
Im pretty interested in what nintendo will come up with as an excuse to return to the pikmin world.