I'd like to have a Bag of Holding, to appease my inner horder.
I've always enjoyed the Easter Egg references to other campaigns/settings in the Baldur's Gate games, so I'd like to see a reference to DragonLance in the mix. Maybe an encounter with a delighted, yet thoroughly confused kender?
Implementation of the various kits in the PHB and the assorted proficiencies that can be taken to augment some of them. Herbalism kit, healer's kit, poisoner's kit, climber's kit, forgery kit. Maybe adding areas that can only be accessed by a climber's kit?

Flumph Fancier. Elsecaller. Tolerated by cats.