If you had to sit down and think of a game idea guaranteed to make billions, don't waste your time thinking of something new. That's madness! Too risky!
It's safer to just throw two existing properties together. Sort of like what Pokémon x Nobunaga's Ambition (aka Pokémon Conquest) is doing, only mix Pokémon with something everyone would want to play.
Plants vs Zombies should do it. The mechanics remain the same on PvZ's side, while on Pokémon's side, you're still collecting them. Sort of. Only instead of sending them into harmless battle for the pleasure of humans, they'll be sacrificing themselves for the sake of your lawn.
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Pokémon vs Zombies [oestranhomundodek]