I enjoyed both, Pokémon XD : Gale of Darkness and Pokémon Colosseum immensely. Their storyline's have enough room for a sequel, considering evil never rests in the Pokémon world. That being said, there could be room for improvement or expanding what the game has to offer. Both games were made for the GameCube, and only deal with Generations I-III. Not to mention the Orre region was rather small. With a little creativity, it could be something even greater. By expanding the region, you improve the population of various Pokémon by including Generations IV-V as well as the other three. Console wise, if there is a new release, I think it would be a great idea to release it on the WiiU. It would hopefully be an excellent starting point for future installments to work off of.
P.S. I'd also like to be able to select my protagonist's gender if they make another game like XD and Colosseum. It's not that I hate being a guy, but I would like a little more selection.