One interesting plot twist could be that Soldier 76's time - traveling abilities (if we assume some extended lore) bring him to the RWBY universe during a crucial moment in a battle. His military tactics and advanced weaponry completely change the tide of the fight. Another could be that in the crossover, Soldier 76 has some sort of connection to one of the RWBY characters' pasts, like he was involved in an event that shaped their family history. And a third plot twist might be that his healing abilities become a key factor in saving a major RWBY character from a near - fatal injury, leading to a new alliance between him and the RWBY team.
A possible plot twist is that Soldier 76 discovers a parallel between his own world's problems and the Grimm situation in RWBY. He then tries to implement the solutions from his world to deal with the Grimm, but they backfire in unexpected ways. For example, his high - tech barricades attract a new type of Grimm that is attracted to technology. Another twist could be that a RWBY character discovers Soldier 76's weakness, which is something completely different from what is expected in his own universe. And finally, a plot twist could be that Soldier 76's visor, which has some special functions in his own lore, starts to show visions of the future of the RWBY world, which he then has to decide whether to act on or not.