I believe it would be very interesting...especially in drags online, to have at minimum engine swaps. There would have to be several different engine options though to keep it interesting and surprising to others (like a Miata with a LS, rotary, 302, etc.). Realistic handling physics would be cool but that would be a lot of work. That being said, I think one reason things like this aren't done are because the majority of the gt5 fan base is too critical so PD just decides otherwise. After reading most of this thread, I have a question for the appropriate people. If PD did include engine swaps at some point, but didn't incorporate correct physics (weight distribution change), would you not use this add on and consider it a negative aspect of the game?
I know EA did this with shift 2 and I thought it was pretty cool. They didn't have much of a variety of engines though so ("I wonder how fast that guys car is") kinda went out the window as you already knew. Im curious as to how Turn 10 did it and how interesting and correct they made it.