All of this is just my opinion of course
It's all nice and great that DirectAdmin has written the entire controlpanel in C without many bugs and no security issues that I can think of. But in my mind they could increase efficiency if they just used php in the frontend. Sure, keep the backend in C, just serve the php pages (which could be edited) like a mini apache server. It would basically be like it is now except that instead of loading hardcoded pages from within the binary itself (+skin files) it would just be a regular php file with tokens.
Lets just say, I think that they could hit several issues/requests with one stone (complete language, access to all links in skin, and more precise plugin placement). Frontend maintenance costs would also probably be reduced. (DirectAdmin wouldn't have to be recompiled for skin changes).
Basically to sum it up. It could be something like DirectAdmin's API. DirectAdmin servers the pages, specific calls in the php file trigger actions like the api, for example creating domain names.
It's a little more complicated than that, but I think it would save them in the long haul and increase functionality and a more editable design. (Unfortunately not modular still)
Just my opinion of course.