I don't believe they can do both. Sadly, if they went this route, I see the same scenario playing out for GT as it did for Forza. These are two totally different genres of racing games and they should absolutely co-exist. Publishers go where the money is though and a title that appeals to a broader audience will obviously sell more and people in suits will say.. why are we even bothering with this other game?
This may come across fairly negative, but I don't think Kaz/PD would make a very good open world racing game. It feels like it'd be a huge learning curve for them, as they still haven't been able to nail down the GT series as it is after all these iterations.
As for Forza Horizon 6, eh, I'm curious but don't feel compelled to buy it. Like with many games that have been turned into a formula to churn out titles, there are diminishing returns for me. I believe I picked Horizon 5 up at launch, played it for a bit (70 hours apparently) and was done with it. Didn't really feel like I enjoyed my time with it as much as it was just something for me to do. It's become so same-y and yet I'll go back to the first one and there's still something different and alluring about it. A simplicity? Or the environment? I dunno.
A real thought I keep having though is that the Horizon series is usually a good way for me to discover new music. I'm not a big music person and I don't seek stuff out or whatever. So often games or some audio clip on an Instagram post will be where a song catches my ear and then I go download it. It's nice to find new stuff. Horizon is one of the few places that has had like dance/electronica/whatever type stuff.