From my experience (and that of friends) the Forza Motorsport series peaked with FM4. It also coincided with the GT franchise losing focus and quality control/performance issues. Today however the Forza community heavily favours Horizon. Which is a closer competitor to Need for Speed than GT.
I agree, I have played most of the FM games (FM5 being the only one I missed) and FM4 was hands down the best for it's time. FM7 is frustratingly dull and lifeless, it's got a cracking car list, a decent track list, but when I play the game I don't feel compelled to keep playing. I fire it up every once in a while thinking I'd love to drive that car for a bit, do one race and I'm done.
It's not a bad game if you look at the individual components, but IMO they are put together badly. There's very little motivation for me to play on, no rewards that I want to achieve (the Forzathons are rubbish and force you to play a certain way), the Forza Edition prize cars which offer perks for using them in a certian way is not an idea I like.
On the one hand FM7 is a game that has been positioned as a gamr you can play how you want to play, and it has a great free play mode, but the cards, forzathons and forza edition cars all fly in the face of that in a rather contradictory way and say "play this way for x reward", it's a muddled mess FM7 IMO.
Gran Turismo probably peaked at GT4 in most peoples minds, however I think I prefered GT3 over GT4. It had the more interesting career mode IMO. GT4's had more different events, but GT3 made good use of every car and track.
It had events repeated across different leagues allowing cars too powerful for the lower league event to compete against faster cars. For example, the Beginner, Amature and Professional FF car races, the beginner ones could be won with your first car, the professional event needed a highly tuned car.
It was almost perfect, the career followed the path of tuning your car, sure you had to use other cars but your car had a place nearly all the way through. It felt like every car had plenty of places to use them competitively, exclufding the Formula GT's which were only comeptive in the Formula GT series of course.
GT4 had more cars and more events, but there were a heck of a lot of cars that felt useless, particulalry in stock trim, GT3 had that nailed. Like GT3, Forza 4 got almost everything right, the variation in the events, having events for simialr types of cars but in different performance classes, it didn't shoehorn you into doing X number of drifts in a race in Y car for Z reward like FM7 does. It was great. FM7's career mode iby contrast is dreadful.
I'm far less interested in games in general now than I was, well not less interested but less involved in them, and that certainly is playing it's part in how I view them, but I can still fire up GT3 or GT4 and enjoy them and I can enjoy them for longer bouts than FM7. GT Sport is a more focused game than FM7, but it's far from perfect. All of the games have pro's and con's, some are hard to put into words, like the way a game feels, it's polish or X factor.
Regardless, if GT7 can combine a great career mode like GT3's with a car selection like FM7's, a great track selection and the Sport mode of GT Sport, I think you'd have a pretty damn good game that should keep people hooked for a good while.