The Gold Ball which increased the capture rate based on the ammount of money you have.
Named for the term "Golddigger".
That reminds me of the
Gil Toss move you can learn in
Final Fantasy - Crisis Core which is based on the amount of Gil/money you have.
That would be sweet and that would really match the Trainer class called Rich Boy. :)
I would love an
Event Ball like someone else mentioned to capture event-only Pokemon in.
With that said, I'm also something of a sentimental fool so I really love the idea behind Cherish Balls and would love to have Poke Balls that could really show my caught Pokemom how much I 'love' them.
Maybe a
Bonded Ball that would auto-heal + auto-HP-replenish a Pokemon that is returned to the ball. The auto-heal of status problems can be automatic, but how much HP it heals could depend on how Happy the Pokemon is with full Happiness = 100% HP replenish (thus a measure of the Bond between Trainer and Pokemon).
A
Speed Ball that would guarantee that your first Pokemon in your group during a battle situation (in-game or otherwise) would be able to make a move first irregardless of your Speed stat would be awesome.
A
Custom Ball which could just be like a regular Poke Ball whose design can be changed by the Trainer.
What I actually would love to be able to do is to switch my Pokemon into different types of Poke Balls. It feels a bit weird to have my favorite Pokemon in regular old Poke Balls when some of my other not-so-favorites got caught in things like Great Balls or Ultra Balls.
The Shiny Gyarados in a Premier Ball is excellent though. I'm so pleased to have caught him in that because the combination of his redness and shiny-ness and the way the Premier Ball opens to release it is just awesome.