This is something I have thought about too. Since I am in the city - there's no need at this point as its rough driving the ///M at times. I always wanted an F-car...and more recently drooling over an Aventador...if moneys not an object, why not? but thats not usually the case.
I think I'd rather try out a Car Club that has an annual membership fee but allows you to drive different exotic cars without the hassle of maintenance. My girlfriend's co-worker does this and I've looked into it before and not such a bad alternative. That's something I might be interested in doing 2-3 years from now. Once I move to the suburbs in 4-5 years I would consider having an exotic if I was going to put at least 400-500 miles a month on it. Preferably a used F430 spyder for a sweet weekend car
The M3 will always have it's place - I debate trading up for an M5 or M6 but just performance wise, the extra weight isnt more fun. I'd rather keep the ///M and move it to a project car once I have a bigger ///M for daily driving (That's if I have the space...but then I feel like having so many cars would become a hassle in itself...inspections, oil changes, car washes etc)
In an ideal world - you get a 5 car garage, a nice daily driver, an exotic, an suv, the wifes car and a wild card...let's hope for the best!
That was a mouthful...but just laying my $0.02 out there.