Maybe it would be safe, but there would be a decent chance you would accidentally mess up sometime simple and makee the algorithm trivial to decrypt. How do I know? I've experimented with variants of hash functions and seen that happen.

IIRC Groestl if you switch the inputs between the P and Q functions you'll introduce fixed points ino Groestl. Or take your example of AES, if you changed AES such that the loop which ran shift rows and increase it to run four times, you'd massively damage diffusion and probably have a trivially breakable block cipher. Modern cryptographic primitives are very carefully built, minor changes can be disastrous.