Please implement the jabber:iq:gateway protocol
Gajim doesn't support the jabber:iq:gateway protocol (part of XEP-100).
This would be very useful for interaction with gateways whose mapping between legacy addresses and JIDs cannot be resolved with JID Escaping.
In terms of user interface, one implementation possibility would be to replace the JID input field and label with the ones suggested by the gateway. Another possibility is to provide a separate zone for translation, much like Psi does. The former is prettier but doesn't allow direct JID input, while the latter is more flexible but adds more weight to the interface. (There are might be other cool presentations, though.)