I can't seem to find the optional transformation, i'm not sure what i'm missing and/or not doing
The optional transformation is part of the cut scene that plays when the last transformation stage (currently) is achieved.
Currently this is how to get transformations
Major transformations (visual changes): Lose to Bosses
Optional transformations (anything from gameplay changes to just flavor text): currently it's just what is previously mentioned, however in the next update there are plans for ones you have to seek out and find yourself.
When it gets to that point a guide will be provided to people don't miss out