I think it would be for the best if rally stays in Horizon. Although they have made significant progress from FM7, Turn 10 still don't have circuit racing dialed in yet. Multiple circuits have mesh/surface issues as well as many have questionable kerb physics, 3-wheeling physics for FWD/AWD are worse than FM7 (and waaaay worse than FH5), fuel burn is still very simple and has no multiplier, tire compound restrictions don't exist, the draft is a little inconsistent and they keep breaking bump drafting, timed races still don't work in a way that makes sense, FRR needs a bunch of work, etc etc.
Then on the content side of things, we are still missing lots of tracks, and lots of variants of existing tracks. Assuming they keep with 1 track every 2 months (which might not even be sustainable once they are done "remastering" tracks from older games), imagine how much worse the content drip will feel when we have to split back and forth between rally circuits and road circuits? Same goes with cars, we get like 2-3 cars per month and at that rate they can't even grow their roster of race cars and road cars in any significant way, how are they also going to start growing a rally car roster? No one would be happy, people would complain there aren't enough rally stages or cars to make it an interesting mode, rally guys would be disappointed with how little they do get, and the core circuit racing playerbase would be disappointed with even less circuit racing content than they were getting before.
People criticized the hell out of FM7 for having no focus and not feeling like it knew what it wanted to do, yet we constantly have people asking for rally, drag racing with expanded drag-related upgrades, drifting and even motorcycles...
It also wouldn't do much to fix the game's other issues too, like bad AI, weak career mode with pointless races that feel like they were made with RNG, multiplayer population/matchmaking, etc... Although it would probably only impact multiplayer population for a few weeks. People already avoid public lobbies because they are too rough, imagine how bad they would be for rallycross? Can't imagine people would be sticking around and would just go back to regular races.
To me this is like going into a Burger King and complaining they don't have a pizza on their menu and telling them to start making pizzas. Their kitchen isn't set up for it, staff aren't trained for it, fridges aren't stocked for it, and most of their customers don't go to a burger place for pizzas anyway when they can just go to a Pizza Hut, or Domino's, or Papa John's, or any of the dozen other places that actually make pizzas.
Horizon already does a pretty good job with casual rally and rallycross racing as does V-Rally 4 (although that one's a bit more debatable), DR2.0 is probably like $10 nowadays and has both leaned a bit more "hardcore" for more serious fans, then you have EA WRC and the last few KT WRC games, and on PC a bunch of options like Beam.ng, modded RBR, modded AC, etc. I don't really see how it's a market T10 need to find a way to barely squeak into, and rally certainly hasn't done anything positive for the Gran Turismo series.
Rather than dilute FM8 and half-ass rally into the game, it would be better for them to full-ass it and revive RalliSport Challenge in spirit as Forza RalliSport or something.
