I personally nominate:

Gordon James Specter/James Gordon Specter.

Either one for prominence, as Harvey's DC knowledge and him and Donna's fathers would easily work in tandem in either order. It's insane thinking that kid is 5/6 years old at this point.

I'm also curious about Marcus and Katie's kids and how their relationship with their uncle is considering he's the reason their parents (presumably) agreed to share custody at the end.

Bobby may still have bad blood with Harvey over who financially s them going forward in a novel follow-up. I imagine Katie spilled the beans about Lily (RIP) and step-grampy Bobby the older her kids got, to give them understanding of where they came from. Maybe she wants to move to Seattle with them to start over, and Bobby tries suing her for negligence to get custody out of retaliation, drawing Harvey in one more time to settle his family's shadows, stand for his ed loved one's memories, and stake his sheltering claim in their future.