I like the private asset exchange,i also proposed it in the main thread some days ago.
It would only need the possibility to issue an asset with a keypass,and the issuer would give the keypass to whoever he wants.It would be great for internal things in enterprises,or private groups,or things like for example a company issuing ticket-restaurants in a certain zone.
My prediction is that we will see many different kinds of marketplaces on top of the Nxt blockchain, with different features and rules. The Nxt network seems to be efficient enough to handle a large number of signalling transactions in addition to the normal spending transactions.