you could make a level like a building, with multiple floors, or a stage bigger than the screen, add a little story
Play Ultimate Assassin 3: http://www.games121.com/2011/06/ultimate-assassin-3.html
I'm very much enjoying this iteration of the game - I think you've successfully set the difficulty and timings just right to make it playable. Perhaps it's just me but I find after say 8-10 scenarios I have to stop for a while - just too stressful! If I shut down the browser and come back a day later I have to start from scratch! Is there a save feature anywhere to save where you're up to?