I doubt Diddy Kong would be cut. The DK franchise
may even gain a character if people speak loud enough. As for third parties, I am personally fine with them characters as long as the characters are video game characters by origin, have a Nintendo appearance, and those standards are kept with the developers.
I'm more interested in keeping the main Smash game very good while improving other aspects of the game, like improving menu navigation, better single player experiences, good balancing, making custom moves not only better, but all of them be ready out of the box if returning, and better move designs for certain characters (I'm looking at you, Ganondorf). A great music selection is always a necessity and is welcomed.
For my personal wants...
- More true alternate costumes. Plus, crossover outfits that keep within Smash's limitations and fit the character wearing the outfit in multiple aspects.
- Expanding the Vault similar to the Namco Museum collections on PS1. Make players feel immersed in the worlds of the franchises in Smash as they view the trophies, models of playable fighters, seeing franchise history, talking to the franchise's NPCs, fun interactions, getting costumes and accessories for Miis . Simultaneously, bringing a social hub for Smash players that don't want to fight at the moment instead of makeshift hangout places in the games. Oh, and a quick mode for the "get on with it" players.
- Optional changes in menu looks, backgrounds, and sounds based on franchises in Smash.