Well, first, I think it depends on where you live, what you want to do with it, and what locally accessible riding opportunities present themselves to make enough use of it.
IF I only had the FJR, I think no. 2 would be a dual sport. Maybe a KTM 950, maybe a V-strom -- not sure, since I haven't actively pursued buying one. Since I have an XX (or two), a dual sport would be my bike no. 3 -- ahead of a sport bike or MX bike.
For me, a true sport bike is too uncomfortable to ride all day, and I doubt I could afford to ride something like an R1 very long before the citations piled up and made my insurance unaffordable.
The XX is a great second bike. Big enough and comfortable enough to ride it all day, and with soft luggage, to tour on it (though the FJR easily wins the long distance multi day comfort competition for me), yet faster and more nimble than the FJR for more twisties fun. Or maybe I'm just in love with the way it looks.