Golduck
Typing: Water -----> Water / Psychic
Abilities: Damp / Cloud Nine / Swift Swim -----> Cloud Nine
BST: 80/82/78/95/80/85 -----> 80/82/108/130/110/90
New moves: Power Gem
To be completely honest, I thought Golduck already was Psychic type before I looked it up and was corrected. Did anyone else make that mistake? I mean, Psyduck basically runs on headache-fueled mind powers, I'm pretty sure I'm not the only one that assumed the line was part-Psychic too.
Anyways, Golduck gets the Psychic-typing because in my opinion it should've had it anyways. For a competitive reason, look at Slowbro and Starmie, representatives of the two currently-existing Water/Psychic lines. Both are notable defensive and offensive threats, respectively, so if we want to improve on Golduck (which we sorely, sorely do), Water/Psychic, in addition to being a good typing flavorwise, is a good place to start. Luckily, Golduck already has a plethora of Psychic moves to choose from (seriously, why isn't it a Psychic-type in the first place? Psychic, Psyshock, Confusion, Zen Headbutt, Amnesia, Wonder Room, Psybeam, Telekinesis... and that's not even all of them.), so I didn't need to make any movepool additions really. Power Gem is there because Game Freak seems bent on making Golduck seem like it has things but it really doesn't even beyond the Psychic-typing. The poor thing has a freaking gem on its forehead for crying out loud, even Persian got Power Gem.
The one thing Golduck has (or had) going for it, at least in Gen III, was a unique ability: Cloud Nine. It temporarily stops the effects of weather while the Pokemon is in play. Now, this ability could have been a game-changer if two things were true:
1. It actually
stopped weather, like some assumed, instead of just pausing it, and...
2. Something decent actually got it. (Sorry, Altaria and Lickilicky)
With the weather nerf in Gen VI, some thought that the death of weather teams was near. But as we now know, this isn't true, and weather teams are still relatively common in OU. Enter Mega Golduck; with Cloud Nine and buffed defenses, it is capable of foiling the plans of many weather-dependent Pokemon. Tyranitar and Cradily lose Sandstorm's Special Defense boost. Mega Houndoom loses Solar Power while Victreebel no longer has Chlorophyll, letting Golduck outspeed it and slam it with an Ice Beam. Kabutops loses its Swift Swim boost, too, and Tornadus misses out on its perfect-accuracy Hurricanes. Walrein no longer gets auto-recovery, and... unfortunately Hail sucks too much for me to think of another example. Basically, Golduck can check weather-dependent Pokemon with Cloud Nine, useful in a weather-heavy metagame. Cloud Nine also could help it find a niche on weather teams of it's own, too. Need a Water-type to help deal with other Fire-types on your Sun team? Mega Golduck not only temporarily takes away the sun to neuter the opponent, but also doesn't have its Water-type moves halved by the weather. If you think about it, Cloud Nine is actually quite the decent ability, if only something usable got it as well.
Of course, being able to stop weather doesn't necessarily mean that Golduck will be able to defeat weather-dependent Pokemon with ease. For example, even without rain, switching Golduck in on say, Thundurus, would not be a smart idea. Cloud Nine enables Golduck to stop Pokemon on opposing weather teams that it would normally be able to had weather not been present (see the above Victreebel example). It does not let it defeat
every Pokemon that relies on weather. Additionally, Cloud Nine doesn't stop weather, only pauses it; should Golduck be forced out, weather effects would resume as usual, for better or for worse. Golduck's Special Attack and Speed are boosted to help it hit harder and faster to take out some of the weather-heavy Pokemon it's intended to, while the boosted Defenses enable it to survive a few more hits, as if it went down too quickly the weather nullification would not be anything more than a minor annoyance to the opposing team; the longer Golduck is in play, the more turns of weather it wastes against its opponent.
Golduck really got the short end of the stick when it came to viability. It for some reason never was a Psychic-type, never got Power Gem (again, not particularly useful competitively but still), and never was particularly good at using what was once its signature ability. Mega Golduck tries to remedy all of these problems, giving Golduck a niche in being a unique candidate in surprising and stopping weather teams. Golduck was always a cool-looking duck...platypus...thing...(kappa?). While its many of its common fellow pond-dwellers (Politoed, Gyarados, Azumarill, and let's just forget about Seaking) have risen from the depths to become top-tier competitive Pokemon, Golduck has never really seen the light of competitive viability. While this Mega certainly won't raise it to the level of these Pokemon, it hopefully will give it a unique enough niche for people to consider it when building a team.