Firstly a mashup of a wiki and Etherpad. So you can browse the wiki as normal - but when it comes to editing you get an Etherpad style interface to allow collaborative editing.

Secondly (and there is a big business here if you get it right) is document management storage. Everything has to be versioned and audited now to meet ISO standard. A server that would accept uploaded word, excel etc. documents, let you edit them and create new ones would be killer, you could track versions automatically and place it prominently in the document as well as maintain an audit of edits. Another wiki aspect would come in in terms of discussing the documents (like a talk page) which is massively useful.