Now this is a topic close to my greedy repo heart
,limited wars. My favorite, and perhaps the best grand strategy game ever designed is Empire in Arms. For those of you who never played, it is a cardboard and paper game of the Napoleonic wars, 1805 to 1815. The reason I bring this game up here is its treaty system which would work perfectly in SEIV.
In EiA, you cannot attack without a declaration of war. That rule could be adopted in some form.
Peace can be arrived at either on consent, or by suing for peace. One side in a war has the option to sue for peace once per turn. The other side then responds with either an unconditional surrender or a conditional surrender. A conditional surrender must be accepted, while an unconditional surrender may be rejected. A conditional surrender, if I remember correctly, allowed the winning side to pick two peace terms, while an unconditional surrender allowed the winning side to pick three peace terms from an expanded list of terms (such as lost territory, occupation of the nation, higher reparations than from the conditional treaty chart).
As far as I know, EiA is out of print due to the demise of Avalon Hill, but will probably be reinstated as the New AH grows. However, if anyone is interested in reading an unofficial rule rewrite, it is available at
www.empiresinharm.com.
Such a system could be the inspiration for a fairly simple treaty system, as most of the pieces are already in place. Ceding systems, planets, tribute, protectorate and subjection status, gifts and so forth are all elements which could be part of a treaty system.