Forza should simply not try to please everyone.
A big part of their problem has been that they have been trying to please everyone, and they have worked themselves into a tough spot because of it. They have spread themselves too thin and haven't been making meaningful advances in really any area of the game for several titles now, and it's catching up to them because the demands for FM8 from every subsection of the community have grown to be a pretty big job. The end result is a game falling behind that is starting to feel like an unfocused mess.
They have drawn in people from all levels of car enthusiast and gamer, and while that means success through bigger sales, it also means the community is split up and you have all kinds of demands from all the groups. Using the car list as an example, all parts of it have been complained about, from people who want more supercars, want more race cars, want more vintage cars, etc because we haven't really got enough new content in any one area to please that subsection of the community. Instead of focusing a bit more, car packs are just random things with basically zero cohesion. Usually a mixture of "tOtAlLy uNiQuE AnD FuN" meme vehicles that a lot of the fanbase drives once for a chuckle and never touches again, some trendy sports/super car, race cars with no competitors in the game so they have to be pushed into divisions they really don't belong in, and SUVs and 4x4s that don't really belong on a track and only end up being used for griefing or memes.
More generally, the hardcore racers want more options and depth, the casual racers want better divisions, the single player guys want better career mode, the multiplayer guys want better league mode and matchmaking, the casual general car people want car soccer back and more tag and cat and mouse type games and improved Forzavista, the drifters want window stickers and Fujimi Kaido, the painters want more vinyl shapes and masking tools, the media guys want more photomode and replay improvements, etc. That's not even getting into all the demands rallycross and off road racing and drag racing and so on.
It's so much to do that it's hard to have any faith in them making any meaningful advancements given their track record. We have seen/heard little so far to prove FM8 won't just add some gimmicks and do the bare minimum to justify release and then ride a bunch of positive reviews from casual gaming sites who play so little that they don't even realize how stale things have gotten.
Actually focusing on the motorsport side of things could bring some direction back to the series, and the focus could allow them to improve the quality and depth in a neglected area of the game instead of doing the bare minimum to get by with every aspect of the game like they have done with the last several titles.
Outside of weather settings, they haven't really done anything to improve the quality of the motorsport-y things since basically FM1, but have improved and/or fiddled with nearly every other aspect of the game. One could argue that physics developments are motorsport-related improvements, but the truth of the matter is that the actual quality of the on-track racing hasn't improved with the physics improvements, so kind of a null gain there. If anything, it's gotten worse thanks to the terrible drafting model.
I don't see many people calling for the game to become another hardcore sim. Sure there are a few asking for that, but I think most people just want Forza Motorsport to be a sim-cade circuit racing game that provides good quality racing with some depth instead of Forza Horizon: Racetrack Edition.