Some IPs can survive multitudes of crossovers, while others do feel as if they've become an entirely different experience after adding in skins for celebrities, other popular characters from different IPs, and more. Call of Duty springs to mind, and it's a reality that creative director Johan Pilestedt doesn't want to bring to Helldivers II.
In a recent tweet, Pilestedt said he believed that too many crossovers could "dilute" the experience, making it feel like something that isn't uniquely Helldivers. However, he did then follow up with a list of the crossovers he'd like to see.
Out of the list, Warhammer 40K, Starship Troopers, Terminator, Alien, and Predator seem perhaps the most likely crossovers or those that would best fit Helldivers. Blade Runner stands out as a very interesting option, though, as we can't help but wonder what that would look like.