Given you already have one of the best Japanese inline 4's
I would go for something with a V-twin..
I have been looking at weekend toys for ages. I keep coming back to the aprilia tuono. Will probably keep its value fairly well and that thumping twin will put a grin on your face.
I also have a taste for the XJR1300... love that retro muscle bike look. B-King is another option.. not many were made or sold so again could be a good investment.