Hi Lukas,
This is cool and an interesting topic. Something that I would like is to see wether we can merge the auto-completion with the AST-based suggestion mechanism (right click on a piece of code -> suggestions), that provides suggestions for refactorings and code browsing. I mean, you can see auto-completion as a particular suggestion to add a word where the current cursor is placed.
A somehow related question is to apply type inference mechanisms to enhance the suggested completions. You can check the work done lately on type inferencers for Pharo in here:
Guille