In the case of default on bonds, there's 3-5 options, as I see it:
1. All bonds are equal, in the case of a default, the player pays out everything, nothing, or some proportion of all outstanding debts.
2. Private bonds are preferential to public, Private bonds are usually the result of a country not being able to get a bond through the conventional means (the public auction), in this case they have to go to a creditor or country, cap in hand, looking for money. Logically countries and creditors in this situation would demane priority "class a" bonds.
2a: allow countries to stipulate the class (and hence the order in which they get paid) of all bonds they sell, public or private.
2b: Give public bonds priority over private bonds.
3: governments can pick and choose what they pay and don't pay off. I could see their being exploits from this approach. For instance, governments might only pay back loans to people they like. They could borrow all of a creditors money, and default on it with no consequences.
I personally like the idea of 2 or 2a, I think it makes things somewhat closer to real life. On the other hand, there's also things that recomend 3 as a possibility, namely it's simplicity and that hews closer to the diplomacy spirit. I'll leave you guys to discuss.