The only thing that would improve the coupe for me would be rwd. Engine transplants are a bit like a band losing a key member, looks the same but the hearts just not The same.
That said, staying within the family is not too bad. For example, a 20VT in an Alfa 156 would be fun, especially if seriously tuned.
Also a 2.4 20V Alfa diesel might be very interesting in a Coupe, if economy with decent performance were the order of the day.
What would be so wrong about a turbo V6 Alfa in a Coupe?
Like I said, as long as you keep it in the family.