I can't speak for the eclipse plugin (i've never got on with eclipse) but NetBeans does a pretty good job at autocompletion, and much more besides. All you need to get basic processing applications working is to "create a library" for processing (essentially all you need to do is provide the path to the processing core.jar). Add that library to your project and you are away to go. Supporting other libraries is pretty easy, just create a path for those libraries as you have done for the core.jar. OpenGL is slightly more tricky but if there is interest, I'm prepared to write a guide. You will have to write more code if you use a full on ide as you will be writing pure java not the abbreviated processing script which gets converted to java before your programs run (I suspect it is for this reason it is unlikely that you will get autocompletion in the processing ide).

I noticed that even the editor in eclipse plug-in doesn't have the auto-complete of the java editor...

Which plugin is this? I use Eclipse for creating Processing libraries and am now using to develop a tool and I get autocomplete in Eclipse editor, provided the core.jar file is on the classpath.

 where can I find the roadmap of the IDE, and what about contribution process(outside of the tools, unless the tools API provide integration points to the editor).

For tools development you might start here

As a long-time Eclipse user, I think that the PDE is deliberately kept simple. I mean, throwing something like Eclipse at a beginner will surely turn him/her off.

However, Eclipse is definitely more powerful than the PDE - which is why I'm using it. I tried the Processing plugin just to find that I'm better off using pure Java development including debugging, auto-complete and all the nice things you get.

But you also have to deal with non-existing Processing types, native libraries (DLLs) etc.

Best regards,
Eric