Cut/Leaf Blade/Giga Drain
Stats/Typing: It's a freaking Grass type, give it some real grass moves, like Giga Drain.
Design: It's a lawn mower. It should be able to Cut stuff. Leaf Blade comes from that weird little appendage it has coming from it's mouth, I think it should be able to harden and hit stuff with.
Need: Rotom-C has been overlooked a ton by most competitive players. Giving it Cut would give it an in-game use (you could even give the others HMs too and have an all Rotom team), and Leaf Blade and Giga Drain would help broaden its movepool beyond Leaf Storm.
Heart Swap/Tail Glow
Stats/Typing: It literally is an inferior Manaphy. It even has Hydration. Heart Swap and Tail Glow could be used to give it some sort of an edge in the metagame so that it could get some use.
Design: If its an inferior Manaphy, at least give it some of Manaphy's ballin' moves. I mean, come on, it even used Heart Swap in the manga. Why is GameFreak always behind the times?
Need: You can talk to any player ever, and odds are they have never even SEEN a Phione. Ever. I didn't even know it existed until I was filling up my Pokédex in Platinum. Heart Swap and Tail Glow could really give it an edge (for those that don't know, Heart Swap switches stat boosts and drops with the opponent and vice versa).