Proxima Midnight

There's one member of the Black Order (a.k.a. the Children of Thanos) from comics that they didn't include, and that's Supergiant. Sure, they may have excluded her out of concern for being over-stuffed, but I think that she could've been introduced in Infinity War as the one who tried to attack the Vision as Shuri was working to extract the Mind Stone from his brow. Proxima Midnight could've acknowledged that Corvus Glaive had died after their skirmish with the Scarlet Witch and the Vision, giving Supergiant the opening to attack the Vision during the Battle of Wakanda. Later, she could've commanded the bridge on the Sanctuary II while the rest of the Black Order fought on the ground during the Battle of Earth. (But of course, that's what I would've done, anyway).