I played 4 and wasn't really impressed; Horizon was a bit more up my alley, but having played way, way too many racing games, Forza 4 was just kind of unsatisfactorily floaty. And, actually, having played Gran Turismo 6, it felt like Forza was throwing cars at me more often than GT was (awarding new cars on each "level up" instead of each championship, meaning you could end up with three new rides before a series was even over), even when it was the express purpose of much of GT.
Gran Turismo's apparently throwing out the car collection in favor of e-sports and heavy simulation this time around. The sim-racing market already offers plenty of options, though. There are many, many great sim-racers, but nobody does Gran Turismo like Gran Turismo, and apparently even they won't, now. Their E3 trailer showing them scoffing at the old games and boasting about "burn[ing] it all down, start from scratch", combined with buying out the exclusive license to the Pikes Peak International Hill Climb so no other series can use it, is making me very much less than happy with Polyphony Digital. It's a bold move, but also feeling like a really dumb one, because there are a lot of other similar simulation options that look better so far.
I dunno if the ghost cars got fixed later, actually! That sounds like something worth trying out sometime. Gran Turismo 6 did allow for it as far as I know, but I hadn't really mixed and matched performance tiers of car. And yeah, F7 seems to be on the way for Windows 10 specifically. I eagerly await a demo, at least, to see if the physics feel right, and to hear if the car/track roster is appealing. (At this point, games lose points from me for having Silverstone and the N?rburgring because I'm just tired of doing them.)