Space shooters can either scroll horizontally (such as Gallaga) or vertically (such as Space Invaders). These are another variety of game that can be both very easy to make and can be big sellers (due to their popularity).
For a Gallaga, style game, you'll want to use the Side Shooter template. The speed does scroll by super slow with the template, and you have to go all the way to the right side of the screen to advance the game faster (possibly resulting in crashing into obstacles). So, you'll want to bring your right barrier in about a third of the playing field before replacing your graphics to make the game more fun (as shown in the following screenshot). Also, consider speeding up the base-movement of the game by adjusting the Forced Movement parameters of the world(s).
For a Space Invaders style game, the preset template we use will be Dogfight. It's a super slow game by default. So to make things more interesting, you may want the game to gradually increase speed ...
