This is definitely a feature I am looking for in future GT games, and one of the main reasons I will be taking a serious look at some other titles in the fall.
The drivetrain swaps, I really can't see why they aren't possible (at the very least, AWD to RWD should be pretty easy). I really don't see why PD haven't done this already, as it adds a ton of versatility to the game, without the need to add more cars.
As much as I would love to see engine swaps, I have a funny feeling PD are shying away from this, due to their "perfectionist" nature. Not taking a shot here...but due to the complexities of engine swaps (the most obvious being weight differences, as mentioned - also includes engine-bay size, transmission alignment, etc), I could see PD not wanting to offer this, as it wouldn't be "realistic".
Although now that I think about it, after what I saw with hacking/modding in GT5, most of the tuning is extremely unrealistic...
Meh, who knows. I would love to see it, I think it would be a huge step in the right direction for GT, but I can see it not happening because PD.