Africa!
Places I'd like to see BA going (back) to on a no-logic basis are all those destinations in Africa that they pulled off years ago, for losing money hand over fist.
Dar, Gaborone, Harare, Lilongwe, Lusaka, Abidjan...
Kigali would be nice as well, as would going back to the Seychelles.
And purely for the spirit of adventure, Antananarivo - I've always had a hankering to go to Madagascar ;-)