Thanks for the comments so far!
Regarding the editor, we already had 2 WYSIWYG prototypes, but it became obvious that if we wanted them to work well with modules, templates etc. - it would be really challenging and painful. Moreover, WYSIWYG mode in browser is much better now than it was a while ago (compatibility between browsers), but still it has major problems.
So the first choice is: source-based or WYSIWYG. Or something between, that has support for basic formatting (bold, italic etc.), but leaves more advanced stuff as wiki source.
And there is one more interesting way we could follow, which is IMHO the most tempting one:
Etherpad has a really simple interface (which is good), provides a chat for collaborative editing (might be interesting). Moreover, we could add support for adding our wiki source snippets. But the real power is unveiled when several people edit the same content. No more page locking.
We have been looking at the technology involved and it looks this is within our powers to implement such a thing.
So this is just an idea, but for me it looks much better than wysiwyg editing, is not that much different from our current editing, and adds really nice collaboration options. And removes edit locks, which in many cases are a nightmare, also internally for us.