I have a idea that just sprung into my head this morning. One of two things here... On the higher levels when it gets to repeating phases (1,2,3-1,2,3) add a 4th and different phase? So it would now be 1,2,3,4 & repeating 1,2,3,4 then Bonus? This gives the option to create a totally new phase that is different.
Another thought would be for the new 4th level only or maybe for ANY phases in the game. If a crab or turtle makes it all the way down to the bottom pipe and comes back up from the top again then it comes out with a smaller baby behind it. Maybe just one smaller baby or maybe 3 babies?
Another variation is when you flip over a turtle or crab but can't get to it in time then it flips back over but it also leaves a egg behind. The egg needs to be hit 3 times to get it to fully crack open with a baby turtle or crab. Also, anything that hits the egg during this time, such as other critters or ice will also help crack it open. That sounds interesting.