I would love to do the trip I just did agian and go overland across Africa - out of your list I got Botswana, Malawi, Zambia and Zimbabwe.
I think I'd love to explore more of Swaziland though as I only spent about 2 or 3 days there when I went last.
OR if I was to chose one I hadn't been to before, I think Rwanda would be extremely interesting because aside from Gorillas, I don't think tourism has opened up too much there yet.
The odd thing is though, a lot of the land locked countries have a massive watersource anyway. (like Malawi has Lake Malawi which when we were there we called it "Ocean Malawi" because it has it's own beaches and waves and everything.

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