Just free thought, as you just bend some civilisation to their knees i would assume there should be some immediate priviliges following.
I would suggest you get immediatelly NP points to get some "special" benefits, and minimum points is 500, its like 13-ish turns if you have like 45 points per turn.
Dont think it would break the game, but its like small little flavour
But im curious, if you get diplomat, would it prevent you from putting additional spy in their capitol for like stealing tech? Or would it break the game, if they agree for capitulation, while
you already have a spy/diplomat in their capitol?