Definitely another Pokémon Mystery Dungeon game. Mystery Dungeon: Rescue Team DX was good, but it had some problems. For one, when I first saw it was coming out, I originally thought it would be an entirely new game, but it turned out to just be a remake of the original Red Rescue Team and Blue Rescue Team Mystery Dungeon games. It had improved some of the features, but it still had a lot of the problems the original games had. The number of floors in many of the dungeons were ridiculously long, and your character's stomach would keep getting empty. Combine these and you get a very obnoxious recurring problem for much of the game (that's actually why I haven't got past the dungeon mission with Groudon yet, and probably never will). Also, and this could just be me, it seemed like there were a lot less Escape Orbs in DX then in the originals.
Personally, I thought the 3DS games Gates To Infinity and Super Mystery Dungeon had solved a lot of this, although Gates To Infinity wouldn't let you recruit legendaries other than the two of the Sword Trio and Super Mystery Dungeon had making connections with other Pokémon rather than recruiting them. The dungeons didn't have 80+ floors, and Gates To Infinity for the majority didn't even have getting hungry as a feature. I just don't find it very fun to have to play through endless floors whilst repeatedly getting hungry and using up all my moves to eventually faint and lose most of my held items.
The three other DS games: Explorers Of Time, Explorers Of Darkness, and Explorers Of Sky were major too- it seemed easier to recruit Pokémon to your team (including legendaries!) and from what I remember you didn't have to buy all the habitats for recruited team members like you had to in the original games. I remember in Blue Rescue Team repeatedly having to go through the dungeon with Groudon, save at the rest place, and go on to fight Groudon hoping to recruit him- only for me to defeat him and nothing happen, and then I would have to exit the game and start again from the save point.
I really hope we get a GOOD new Pokémon Mystery Dungeon game soon.