I doubt much of the Peoject Hail Mary fandom will see this, but since we're making The Ultimate Crossover with PHM and basically any other space centered (and trapped in space) media, I think Grace and Rocky should meet Robin and Al-An from Subnautica: Below Zero.
We can even throw in "abandoned in an alien ocean by their governing corporation with little hope of surviving and escaping on their own" Riley Robinson from the first game to pair up with "Abandoned in an alien ocean to die by their governing corporation without hope of surviving and escaping on their own" Simon from Iron Lung for any BloodyMary appreciators.
Two humans with their incredibly advanced, techy talking, hard exterior aliens that constantly call them inefficient despite their fondness for the squishy humans. All paired with the guys that had to survive the horrors solo, realizing they're not the first ones down in the depths but are the only ones alive, fighting the horrors tooth and nail. Except Riley got the good ending of actually having the telepathic vision inducing giant alien being nice and helping him get out.
