With regards to an Android 18 bodyswap I'm thinking of getting figma motoko kusanagi when she comes out.
Failing that figma Adult Mikuru may be another option.
Hopefully a figuarts head swap is a possibility.
First Preference Metal Cooler
A fan of the character design, the metallic look could be eye catching if done well.
Second Preference Kid Buu
As others have said need to finish off the main villains, could have interesting accessories such as a stretched arm.