I personally loved both Colosseum and XD, and beat both of them twice. Seeing a third installment, maybe more based upon Pokémon unobtainable in the sixth gen games from other generations (as was the point of Colo/XD for R/S/FR/LG/E) could be implemented. Also loved the original Stadium 1&2, and I think the return of the battle on the big screen would be much appreciated for expansion into a household multiplayer, as opposed to always being on two screens, or using Wi-Fi to battle people with Action Replay'd Pokémon that most people don't stand a chance of beating.
Never personally played Pokémon snap, but I saw Marriland participated in a form of Versus of it with another guy on YouTube, and it looked fairly interesting, and quite possibly worth an attempt on their part for sixth generation.
Other than this, though, not much holds worth for me. PMD has always been "HALP, somebody take thing break universe come to universe and no break!" and I've played the same story six times now (Red, Blue, Time, Space, Darkness, and Gates). If they're going to do it again, please for the love of Arceus, change the story. At least a little? Please?