What do you consider to be "fun" ?
What do you consider to be fun? Start with two distinct, well-known and popular games, combine their gameplay elements in unexpected ways, prototype until you get something that looks like a game that could conceivably be played on a mobile device, get people to play it, collect feedback, iterate to fix any major flaws identified by your testers, repeat until you hit the deadline, submit, profit.
Fun cannot be calculated (no matter how hard some people would like it to), it just "happens". So get coding and surprise us
Here's a few ideas to get your neurons started:
- bomberman and tetris (perhaps two players fighting against each other for blocks somehow)
- frogger and snake (the snake can be killed by moving objects - how?)
- FPS elements in a top-down 2D game controlled by tilting the phone - think paintball?
Also, polish when you've got the gameplay sorted out. Good art, good music, everything, else your awesome game won't score very high, unfortunately.