Well, the E30 M3 is an iconical car to say the least. The Silvia... not so much. Most japanese car enthousiasts (especially drifters) would love to see this car in the game but the chances it will be a likely zero. It doesn't bring anything special compared to the other cars in the game. And they're not that great in stock form...
A GT86 would have a greater chance to be in the game then the Silvia, but would not really be a addition to the game for me. Don't get me wrong, I've driven the car for quiet a time and love the handling etc... But I find it too slow on track.
And it's also pretty cool that they stuck an E46 M3 engine into the E30, would be my choice in real life too! (or use my M3 engine)