Introduce ExitNode country code per listener
Born out of this tor-talk reply you could introduce the ExitNode country code per listener.
It could be used for a listener that listens for Thunderbird traffic. E-mail providers with restrictive policies, which lock you out because the connecting IP is not from a country you are normally in, could be used over Tor with such a per listener ExitNode selection, without having to use the same exit set for browsing.
The summary limits it to the country code, which should enhance the user experience since restrictive service providers should work over Tor, however I would not mind if this feature is similar to the global ExitNode option.
The global option is more flexible and would allow to select only exits that are known to work (e.g. one that are not blacklisted by the service provider). However it might be problematic since clients are bound to certain exits. Depending on how narrow that set is it could degrade anonymity to a high degree.