I've said this before as well, but for Gracie, I've love to see one of the children of the other 4 Dons (of the Big 5 Families, which includes DeLucas).
Perhaps a guy, who is highly intelligent, yet he is smug, arrogant. He plays in some ways like a mastermind like role doing things behind the scenes, and thinks of all the other Families as if they're his pawns to play around with.
Gracie could initially have battle of wits with him, and loses the first time. But with support from MC and Luna, gets back up and wins in the rematch.
Seen some people say Wilfred should be Gracie's rival, but I'm not a big fan of that idea tbh. I think rivals (not just Gracie, but for every DeLuca character who might have a rival) work best when they're of similar age (or in similar age group, i.e. same generation).
Plus, Gracie's at the point right now where she thinks no one can outsmart her, at least in her age group. Having her stumble (an experience that she never went through before), and get back up I think will really evolve her character as a whole.