Assuming I have full creative control:
- Fully 3D worlds. None of this fixed camera thing, now the player has full control of the camera. Environments are designed in such a way that there is no missing which direction is progress, whether you enter a room facing the exit, the exit is lit up and easy to see, or (in combination with one of the others) the camera automatically points toward progress after a full minute of nothing happening (no coins picked up, no enemies defeated, no collectables collected).
- Elemental zones are back to being unlocked by any Skylander of that element. Elemental zones all have a puzzle or a battle in there. These puzzles and battles are much trickier than the ones on the overworld - enough so that it is possible some kids won't be able to get all of the hats by themselves.
- All dialogue and cutscenes are skippable, and skippable without hearing a single word (no "are you ready-"'s). If you accidentally skip some important information, you can talk to the relevant character and they will summarize the important parts.
- The upgrade menu is Swap Force-style. After each upgrade, you go right back to the menu so you can upgrade again. No more talking to Persephone more than necessary!
- Puzzles would have multiple solutions, taking each character's abilities into account. For example, say there is a treasure chest on a nearby island. You could find wood to build a bridge with, but you could also take a skylander who can fly (or one of the Rocket Swap characters) and have them fly over the gap. Or you could use a Giant to pull the island over to you. Another example is if you are blocked by a wooden barricade. You could find a bomb, use a bomb attack (ex. Boomer), use a Giant to crash through, or burn it down.
- There would be no game-specific gimmick, but there would be new characters for each of the previous games' gimmicks. There would be a small amount of reposes for each of the previous games' gimmicks as well. (Two per game? I know it's not even with the elements, but TFB obviously didn't care about element equality with Trap Team's reposes).
- For cores, there would be one new character released, and two older characters re-released per element. One of which is a "fan favorite" (Spyro, Jet Vac, etc) and the other is one who hasn't had a repose yet (Sunburn, Fright Rider, etc.).
- PvP returns, and also has an online option. Online PvP has no headset support, so players cannot talk to one another. There would be at least two modes: free-for-all and collection-only (the latter is so that someone who doesn't have all the best characters doesn't feel left out). Collection-only mode is hidden away in the options menu, so free-for-all can promote sales. (Gotta stay on Activision's good side

)
- Taking inspiration from Swap Force, there would be ability-specific minigames accessible through the hubworld, and unlocked as the game progresses. They would mostly involve the same abilities Swap Force did, but more characters can play them after you get certain Legendary Treasures (like regular flying characters can play in Rocket zones after you get a speed star Legendary Treasure (so totally not a Spyro reference)). There would also be minigames for fire-breathers, lightning users, etc.
- Oh, and said minigames in the above would have an online and in-home multiplayer option. You can either compete for the best score, or see who can finish faster.
- Villains from Trap Team would be playable, and new villains would be accessible too, but would only appear if you have at least one Trap Master in your collection. No new traps would be released, but if you don't have any traps, you can tag-team any new villain with your Trap Master (and only your Trap Master). (I really don't know how to make the Trap Masters special otherwise).
- All old Location pieces would be usable again, and they would all be re-released and repackaged, either with a new character, or a repose of the character that originally came with it (these characters would be released separately, like in Swap Force and Trap Team). All of the areas would be redesigned with the new standards in mind (see first bullet point). Any collectables or other unlockables unlocked in these areas would not appear in the menus until the location piece is placed.
Is that it? I think so. Of course, most of these would never happen, since Activision cares too much about sucking the money out of people's pockets. But hey, see the last one. It could get pretty expensive if you don't have them already