I got a chance to play your game to which it reminds me little of Metal Slug meets Gradius. If I add a suggestion is when upgrading a weapon have the character change color or costume that is based on a weapon. Next when playing two or three players have them combine into one for 10 to 15 seconds. The first player can control the movement as the rest of the players can shoot rapid in a circular spread bullets, whenever they acquire an item like a glowing star. I hope these suggestions would help your game.