I'm going on with my experiments. After learning how to insert advertising in my app (wow: I gained 1 cent in a week: D) I'm continuing to change the game. I changed some - small - graphic details, Including banner on Google Play. But there are two big changes. And one little.
First: I added the chance to continue the current game. You can choose to "pay" for this possibility with "rubies" or watching a video advertisement. I think "25 rubies" is a reasonable level: you can pick up in two or three games.
Second: Rubies. I'm trying to learn how to manage a "virtual currency" in the game. I think to use them to allow the player to buy super powers. Or to change the game's skin. Or to unlock another level. I do not know yet, but I'm learning to handle this addition.
Last: I added two more extras in the game. With the first, the Player loses 5 more points. The second, blows up the enemies closer. And I'm adding some "special effects."