Thank you very, very much for the input! :D
Actually, there is some variation between the stages. The squares are spawning ever so slightly faster every few seconds. And depending on the stage the initial spawn time varies. But if it's not really noticeable then I should definitely tweak that. In the last stage you also get a doubled paddle/ shield. And I already thought about making the paddle smaller or bigger. I was also thinking about different enemy types or bonus/ malucious squares that would trigger different shield types, explosion of all squares on the screen and what not. Different attack patterns though didn't come to my mind, yet. Great idea!
And lo and behold, today I played with glow effects of paddle and squares that illuminate a darker background! :) Haven't touched those effects ever before and still struggle a lot with them. But I agree that the visuals can be much, much more improved. Besides level design that's my weakest point. So I even more appreciate your suggestions, which are really very helpful!
Thinking about it, I totally agree on not mixing styles. Font will be exchanged. And considering the rest: October just has started. ;)
Thanks again and having a look at your other games: I'm definetly learning from a master of the art here! ;)