What I have done is expand out the metamorphosis seed that appeared in Legends of Athas by increasing the number of factors, and making metamorphosis do whatever is in the books and the game. The various DCs that I have there are open for debate. Also there may be some thing that I have forgotten. Also I have some differences with the folks at athas org in that I think that to cast metamorphosis you need both epic spells and epic powers. You can ignore that if you don’t like it.
Additionally, I place a limitation on the metamorphosis seed so that it can only be used a limited number of times. Reproduced below.
Special: This seed can only be used on a single character or creature a maximum of 11 times total as there is only a certain amount of strain of metamorphosis that the body can tolerate (and it is also the reason why Kalak and Abalach-Re seek shortcuts in the metamorphosis – it is beneficial to get the most out of each metamorphosis since each individual can only be affected by metamorphosis 11 times total).
Here is the link to the PDF that I created. Looking forward to feedback.