That was sort of the point of
Eelektross' line, though. It was meant to LOOK like a fish but not actually be one. You don't even encounter them in water, you encounter them in a cave. They float, and swim through the air -like- it was water. Them being pure Electric with Levitate means they're not weak to ANYTHING. Make it Electric/Water, and you completely ruin the one thing that makes it special.
In all fairness,
Gyarados is based on a dragon wind-kite as well as the dragon legend. Those long, swaying kites that look like dragons with wide open mouths? That's the origin behind its appearance. I agree the Mega should become Water/Dragon, though, and I think the normal one SHOULD get some more Flying moves.
From what I've heard,
Mr. Mime is based on a japanese elf or something, so I guess that explains the mystical implications Fairy gives? Kind of weak reasoning, though. I personally feel like it should have remained Psychic pure.
I agree on
Lugia being Water, but I feel like it shouldn't be Psychic. I get that it has Psychic capabilities, but it's a gigantic legendary -BIRD-, specifically connected to Ho-oh (another bird) and the Kanto legendary birds. The "Flying" typing is pretty much mandatory. Psychic should have just been a tertiary theme to it, not an actual type... and Water/Flying is a great typing, anyhow.
Some of my own...
Luxray: Should be Electric/Dark. It has distinct, iconic characteristics of a Dark-type, and it even learns a decent number of Dark moves. Being Electric/Dark would make it better because it'd give it a lot more resistances and even an immunity. It'd also give it Dark-STAB, which is always nice to have.
Electivire: Possibly Electabuzz as well, but Electivire NEEDS to be Electric/Fighting. It's a giant gorilla-cat... thing with Popeye arms and massive punching fists. It battles BY fighting in a very FIGHTING-type way. Being Electric/Fighting would be an amazing, UNIQUE typing that would actually make it somewhat usable. STAB on Electric -and- Fighting? That's scary.
Celebi: I get that this would actually CHANGE a present type rather than just adding one... but it simply makes no sense why Celebi isn't Grass/Fairy. It's basically THE fairy Pokemon in terms of its design and origin. It's by far the most "Fairy-like" Pokemon in appearance besides maybe Flabebe, and Fairy is a MUCH BETTER TYPE than Psychic. It'd actually become pretty powerful, so long as it got Fairy type moves to accompany the change.
MEGA Blastoise: Why aren't you Water/Steel? Even your normal form has METAL CANNONS sticking out of it, but THIS version specifically goes out of its way to become more like a tank... a literal tank. It hunches over further, gets a big cannon in the center, and becomes bulkier. There is NO reason it shouldn't be Water/Steel. Maybe it'd actually make Blastoise a genuine threat if they gave it this typing.
Finally...
I think Yanmega should really be Bug/Dragon, it being a dragonfly and all. It just makes sense for it to be part-Dragon!
This. So very, very much this. The fact they decided to give Yanmega the most GENERIC, not to mention AWFUL typing for a bug instead of the obvious, far more fitting, and UNIQUE Bug/Dragon typing is beyond me.