SuperHotLine is better than Shootout. It is simpler, more enjoyable, not overdone but just right. Pity there are so few levels, perhaps the game would also use a new gun or two. We have straightline singlefire (pistol), straightline rapidfire (rifle), scatter singlefire (shotgun), what we need is scatter rapidfire (automatic shotgun). Maybe some grenade launcher would do, with bouncing off the walls mechanic, I know you have one in the Shootout. It may provide some ideas for extra levels. Alternatively, perhaps some laser gun, the fire of which could bounce off the walls, unnecessarily causing an explosion effect, if the latter one is troublesome, gameplay wise. The laser could also bounce off the glass, say, not smash it down. Interesting effect would be the splitting of a laser beam, making it a chain reaction in some cases.
On a sidenote, on the Linux, I encounter a small problem, namely, the game takes to launch only from the Terminal and spits some minor errors in the feedback but does work without any further issue.
EDIT:
Also, a challenge system: finish the game in maximum X minutes without getting defeated even once.