Well, it makes sense to get something that's very different from a sportbike, right?
What you should get kind of depends where you live and what sort of riding you like to do. I have a '00 Daytona, and since I live in NYC, I recently got a used Suzuki DR-Z400S as a city bike. You can't beat a light dual-sport on bumpy, potholed city streets.
If you live anywhere near off-road trails, a DR-Z is a no-brainer, too.
But if you like long trips with the wife, a used FJR or GS would be a good choice.
The bottom line is: since you already have a sportbike, get something very different from what you already have. There is no point in getting a Monster or Speed Triple when you have a Daytona.