That is a tough question. My 135 is paid off, and I don't plan on getting rid of it anytime soon, so I would probably just add another car to the stable instead. My ideas for another car are all over the board though as I would consider:
BMW E39 M5
BMW Z4 M Coupe
Alfa Romeo GTV ('67 - '76)
Porsche 911 (930, 964, 993)
Ford Mustang Boss 302
I'll also be interested to see what any potential BMW M2 or Z2 will be like if/when they become available.
Delivered in Munich, broken in on the Nurburgring.