This is early days, so my worries may be premature, but i worry Grip may not be enticing to any popular Youtube channel, due to it being slightly off theme to any of the typical Youtube "Gamer types" that exist.
You got your basic Indie Gamers. You got your Xbox and Ps4 game series players. You got your Minecrafters and Besiege players etc...
You're probably right that our best hope is Indie gamers like Markiplier and JackSepticEye perhaps, but they usually play basic-type horror games and comedy games. It does feel a little off from even their usual theme of games they select and play. But after all, they do play things like Rocket League online, so our best hope would probably be getting them interested in playing the Online modes of Grip (Probably modes such as Arena mode and Soccer, which is why i keep arguing this is such an important feature of the game that should be implemented, but hopefully also online race mode, with the enticing feature that they can race and beat their fellow Youtubers online, as i have noticed with certain online games there is a compelling lighthearted competitiveness between small groups of Youtubers which make then play the game more).
I can't see people like Markiplier actually doing a series play-through of the career mode. That's too off from his usual theme. It would definitely be online gameplay against others and his other fellow Youtube buddies, like Bob and Wade. That's our best opportunity, and thus why i think, again, the online modes and experience is going to be so important to prioritise!
No doubt there will be small Youtubers releasing Grip gameplay because they stumble upon it and like it, but i do predict it's going to be a problem getting good Youtube publicity, and so this will have to be approached and presented tactfully by Chris and Rob.
But that's just some of my fleeting thoughts put into a comment and i could be completely wrong! i'm just a random Rollcage fan with no advertising / marketing experience!