The closest thing I can think of is that I want a few Mythos with more blanket customizability, like picking any valid weapon property, or gaining an assortment of any spells of a particular level. As a construct, as far as I know, things like Alter Self could turn you into a Warforged or similar, and Polymorph could turn you into a variety of things.
Rubilax and Shark Faced Ninja Guy are kinda begging for a Mythos that involves fusing bodies with your host. That's... kinda similar?