This is something which might add another little bit of choice to the game - one of the ways that you can make a rougher engine (e.g. a I4 or V6) more comfortable for the passengers is by designing a mount that damps out the vibrations from the engine. (I have a hard time finding exact data, but different kinds used in factory carsare.) I figure this is something you could add to the first Trim page, where you select the drivetrain, and a quality slider could determine weight, expense, effectiveness, and reliability. The type of motor mount might also affect the size of engine that can be fitted - a hard mount would take up less volume and require less clearance for vibration than, say, a hydraulic mount - but I don’t know how much of an effect that would have.
That is an interesting suggestion, definitely has the pros and cons needed to be considered, too. 
Unfortunately, at this point I have to say it would be feature creep to implement something like that though.
Cheers!
Fair enough!
Or a counterrotating system like the Delta Integrale.
I think you mean the counter balancing shaft. It was used in a lot of Fiat/Lancia DOHC engines.
Yes. Is this.