Nice Engine. Now finish it!
If you even just had a time limit, then that would at least make it a high-score-attack-type game.
What you should do though is have a time limit as well as a minimum score needed to progress to the next level.
Once you get to the next level, new celebrities could be added to the mix.
Or you could have a certain celebrity you need to kill on each level? With innocents you must avoid?
Also, you could throw limited ammo and ammo boxes into the mix if you fancied...
By the way, I've played a few of your games and they all seem really great.