C++ code completion has so far been a big problem for all C++ IDE I've ever seen. It doesn't even work in VC++ as it's supposed to?
So, my 'suggestion', and that is strictly IMHO, is to write a code completion library, something similar to CTags, which would have a simple to use api and being a stand-alone project, it could have a lot of contributors, an could quickly develop to be very fast and usable.
That way, every IDE can use that library to everyone's benefit;
I'm not sure if this is the best way to approach this issue, this is just an idea I had.
If enough interest is shown, I'd be very interested to start a new GPL project and start working on the project.