I'd like to see a warframe that's a bit more abstract or "weird" in its design. A warframe that deals with things in a metaphorical way.
The theme isn't just the normal "mask", but I supposed that 'mask' will bring the point across better, since masks dabble in a bit more than the physical thing. The notion was inspired by various things that I tried putting together. Stitches and strings or chains. Puppets and dolls or manikins.
The overall notion is that the warframe manipulates the environment by creating various things or enforcing a control over a certain space. Say, placing masks around that, as an example, is connected through different strings. Hitting the strings might create shock-waves. The strings might also act a barriers. The masks could create fear. Or different masks could be representing different themes, like "the faceless mask" etc.
My overall explanation is quite abstract, but I hope my explanation was, at the very least, explained well enough, or gives an overall idea of the theme.