A Wii U stage idea: Gateway Colosseum from Pokémon Battle Revolution. The entirety of Pokétopia can be seen in the background of what could be called the most iconic colosseum of the game.
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To fit the gameplay of Smash Bros., the circular area of the entire colosseum would be reduced in size, resulting a huge donut-shaped bottomless pit between the structure and the actual arena. Platforms could be added to the stage as well. The background would feature fireworks and the cheering of spectators could be constantly heard on the stage, maybe reacting to the battle as well. Flying Pokémon could also be featured in the background.
Maybe this could also have the transforming mechanic like the Pokémon Stadiums from previous Smash Bros. games. Being from Gen IV, this could be Lucario's home stage too.
Some music that could appear on this stage...
Pokémon Battle Revolution - Gateway Colosseum
Pokémon Red / Blue - Pokémon Main Theme
Pokémon Colosseum - Cipher Peon Battle
Assuming Palkia is NOT a Poké Ball summon on that version...
There could still exist a new version of that stage, though...