I'd be glad to enhance eCompletion for 3.9. In fact, I'm working on an new release. It'll include:

1) Improved receiver guessing (including constants such as boolean, blocks, strings etc.).
2) Instance variable type guessing: I looked at the RoelTyper, but unfortunatly I don't understand everything as it is VW specific. To get to know squeak bytecodes I wrote a simple version of a guesser based on the idea of InstVarRefLocator and it works quite well for eCompletion (it doesn't need everything that Roeltyper does). I'll integrate with RoelTyper as it comes to Squeak.
3) Completion will be possible without a typing the first character.
4) Override completion: If you are at the first word in the source, You get the completion of all methods you can override. The completion then inserts the method with all its parameters and a super.
5) Assignment completion: complete and get an assignment to a new local variable.
6) TestCases, BugFixes and a first Model <-> Morph separation to support different UIs.

Please tell me: What is a super cool completion system for you? What ideas are around?

I'm working with 3.7 and don't know about 3.8/3.9, yet. What's the difference anyway? How is eCompletion doing on this versions? 

Ruben