I reckon Megaempress would benefit from a little paintwork on her face. A tiny bit of lipstick (purple of course), maybe some blush...
Regarding the "Which bra for Megatronia" conundrum, whilst I'll probably try and get her on her own and thus won't bother combining her, I'd say the best option would be Wreck-Gar with select parts painted black and a new head (Shapeways to the rescue!). I seem to recall the mold can be finagled into a sort of pistol mode, so perhaps with some further Shapeways upgrades, Megatronia could gain the ability to rip her bra off and use it as a gun, stunning the enemy with her brazenness and making them easy prey.
The new character, meanwhile, could be called Sexpistol.