Well, to be frank, I would not consider myself an arcade racer, or sim racer - I'm a
gamer.
What does that mean? It means I engage in
interactive software designed to allow me to enjoy the challenges and opportunities it presents. Granted, some games' experiences are "cheaper" than others, some barely have any challenge to them at all. Others are frustrating to the point of "diminished responsibility"...
Anyway, what I'm trying to say is, GT has always been a game about driving and racing cars against other cars, on circuits specifically designed / arranged for such.
In the real world, car races exist with entrants sporting cars with engines that were never intended to be installed in them. This is a fact of life. This stuff happens - sometimes it's obviously like a "gimmick" (V6 Corsa?), other times it's really beneficial (Speed 6 Tuscan Cup car, replacing the ageing Rover V8). There must be a reason that so many of the Japanese tuners are throwing stroked K-series motors in their Fits, EGs etc.
It's simple:
the more modern engine serves as a better standpoint for hardcore tuning, owing to the improved block casts and stock components. Even on the road, considerable advantage can be found just by swapping out to a similar family engine - B18s into Civics is really common.
(Excuse the apparent bias to Honda, but it's what I have the most real-world experience with.)
Engine swaps at the very least are legitimate. Drivetrain swaps a little less so. If you're swapping out an engine, chances are you're going to need a new gearbox (that's part of the drivetrain), so at the least, your gear ratios are going to change - especially if you swap out to a bespoke Getrag unit, or whatever.
Swapping RWD cars to 4WD should be severly restricted, as it is in real life. Very few people are interested in making a car 4WD from RWD; FWD on the other hand. Then there's the world of rallying...
My point is, if it does make it in, and if it's done properly (which it may well be), it shan't at all be "arcadey". Open your minds people, GT
isn't Forza / NFS
et al, despite what the majority of its players perhaps may want.
(no, it's not perfect either - but neither are you )