...Why did Mario beome a plumber in the first place?
I think Mario's career changes in some games just to give the games a different atmosphere and a different style of play. For example--to create a tetris game with Mario sprites, you can't implement the idea that Mario is a plumber; what could you "stack" when you're a plumber? Turtles?
Can you stack pills and match capsule pieces with matching colors? Yes, you can. Now, who uses capsules? Doctors. To add this game a unique feel, what should we do? Let's add viruses of different colors, and let's make the objective to kill the viruses with matching capsules. Now you have a unique puzzle game.