Generation 5 didn't have a Stadium Spinoff. Generation 5 Spinoffs are the new Mystery Dungeon to come out, Pokemon Rumble Blast, PokePark 2, Conquest, 3D Pokedex/Pro, Typeing game, and TCG how to play.
If anything for Gen 6, we'll get another PMD and prolly something close to Conquest unless they make another Ranger game.
I don't think Gen 5 got a Stadium spinoff because of its Random Match Up feature. If they kept that feature with Gen 6, I see almost no point in a Stadium for Gen 6 as the Pokemon are now in 3D as a Handheld so I can battle online against people at any Wifi compatible location, not to mention fill the role of standard tournament styled Stadium play with Battle Tower or what ever replaces it.
The Handhelds seem to have stolen the need for the Stadiums.