Somewhat interesting that Castform's your favourite mon...I usually see the little guy in people's least favourite's list...
I personally have nothing really against it, just never really thought much of it other than it having a neat gimmick...
Anyways...I'm closing in on 90% completion, Melemele's almost done as well...
Castform's just always been a staple of my in-game teams since the RSE days.
I used it in Emerald and it did surprisingly well for itself...so much so that I used it again whenever they could be found in the main game. Black 2, Omega Ruby, and now Moon. Almost like the team mascot. Myself Omega Ruby one in particular was great...she had Master Rank in every contest and wiped the floor with Wallace and Lissia on multiple occasions. :D Seriously, Castform is an amazing Super Contest Pokémon, even if it can't battle well.
Still hoping it gets stat buffs at least someday. Ideally here's what I'd do...
1. Major buffs to its speed and special attack, but keep it decently frail to not be too broken.
2. Give it a new Hidden Ability called Stormwatch. This ability would activate immediately, a la Imposter. If Castform has a move in its Moveset that benefits from a certain weather condition, that weather condition will take effect. For example, if it has Thunder, Rain will be called. This effectively gives Castform a more versatile weather setting ability so it can see more usage.
3. If they want to be REALLY nice...give it Fairy as a secondary typing. It's in the Fairy egg group after all. That'd also make Sunny Castform the only Fire-Fairy type, too.