I agree GalCiv II would benefit from having more diplomatic options than GalCiv I. However there is a limiting factor IMHO : the AIs should be able to use all those options reasonably well.
IMHO the most needed diplomatic concept should be a kind of Peace Treaty or Cease Fire Treaty. For a given period of time, both races agree not to wage war on each other. If the treaty is broken before it expires, the attacker should be penalized, for instance by a loss of morale, morality worsening, diplomatic relations with other civilizations hurt, loss of influence, temporarily banned from the UP... Instead of suing for peace as races do in GalCiv I (i.e. the hostilies end but there is no formal binding agreement and do not prevent alliances from dragging you to war again,), you would sue for a given time of cease fire, the longer the more expensive. Alliances against the contracting parties would not work during this period of time, and that could lead the neglected allies to terminate an alliance he now considers useless.
I'd like to see some kind of Trade agreement too, Im' not too sure about the exact way it should work because I don't know how the Trade system will be done in GalCiv II. If the current system is not revamped (and I hope it will), there is no real need for Trade Agreements.