Feels nice, could probably do with one or two extra bits to spice it up.
Interesting screen layout there. A couple of 32-pixel columns for score/lives, so I take it this is to be displayed vertically? You could design special digit characters which are rotated through 90 degrees to be displayed on each side of the play area for a slightly different look.
Any more gameplay elements in mind for later levels? How about one or two of these:
Black holes that suddenly open up in the floor, then disappear
Doors that need to be unlocked with keys
Walls that appear/disappear
Ghost-like enemies that drift around screen, and through walls
A sub-game between levels
Bonuses that speed up/slow down the player
Timebombs to kill nasties
Those flying enemies could be turned into vampire bats (additionally you could have some comedy Dracula-style false teeth chasing the player around), with the idea being to collect items such as garlic, a crucifix, a wooden stake and a silver bullet. Perhaps you could have coffins lying about for the player to open - some might contain bonuses, others more vampires to avoid, and a day/night indicator, with enemies pursuing the player during the night and running away during daylight hours.
Alternatively, the player could be a secret agent investigating a foreign embassy to collect secret documents. Additional bonuses might include different disguises for the player to wear which would change the player's appearance. Each disguise might fool a single different type of enemy sprite into ignoring the player. Power-ups might include James Bond-style gadgets to aid the mission.
If all else fails, just bang in any surreal ideas that you think will enhance the game - walking filing cabinets perhaps? Other than that I don't really know, just use your imagination.