I have a potentially interesting dynamic difficulty system that I'm currently playing with that might do what you want.  It's also linked to the bonus multiplier so it makes for a nice risk/reward system.  Hopefully

Risk/reward system sounds exciting!

It was a classic developer assumption.  [...]

Just to clear up, the amount of damage an enemy does in the current version of Echoes depends on how much energy it has left.  So a new Black Hole might do 20 points of damage, but one that you have almost destroyed might only do 1 point.  In the new version the damage done is fixed so things should be clearer.

Wow, I am impressed by the damage system you described... Sure it took a lot of work, which, because of the classical developer assumption, i never could appreciate up to now .

What came to my mind is that if you want to keep such a system you could fill the shapes with color, and let the color blur away if they have been hit/will cause less damage. I'd suppose that could work pretty well, tho it would change the look of the game considerably.

You want this then
http://www.binaryzoo.com/forum/index.php?topic=993.0

Exaclty that is what I wanted!! Cool game. Thanks

I am a big fan of echos and I played it a lot. Exciting that you make a new version of it.

I'd agree that it does not need a lot of changes. But I think you should consider some changes to the flow of the game. Losing a lot of weapons has been mentioned as a problem in the forums. I also think it would be great if the enemies move at different speed depending on difficulty level, or, in a perfect world, on in-game performance of the player (so he can tweak difficulty whilst playing - I am refering to the concept of e.g. the police in GTA, where the player can control the difficulty by controling how many stars he wants...). I think what would be even cooler than new weapons are new extra items, e.g. an item that slows down some specific kinds of enemies for 3 seconds.

Communicating to the player what happens: It is not quite clear how much points you score when doing what, so some hints for the player would be great. I think you should also communicate better how much damage is dealt by enemies. Sometimes I lose only 3 points, sometimes 5... despite playing the game a lot I have not really understood (neither reflected a lot ) by which enemies different kinds of damage are caused.

I like your achievement system and I think you should add more achievements. Then also make it easier to unlock crack. I haven't unlocked crack, despite uncountable hours I spend playing echoes (and losing my progress as the profile got lost again )... But I figure you could have e.g. different difficulty levels in crack. So you could unlock the easy version of crack with e.g. 33% percent, normal with 66% etc...

It is a great game, it will be a great game
all the best with the new version!