I remember reading here about wishing to keep the princess, but it was mentioned that the Romans just didn't use that type of diplomacy so have a new idea (well someone probably has suggested it), and one that should be really obvious to all of us nerds.

Instead of being an extra diplomat, why not make her majesty an extra merchant?

When the men of the ancient world were away, or occupied by the Cursus Honorum their wives generally looked after their estate/ordered their slaves around, and they would also have to look after their share of the inheritance from their paternal family.

This seems to fit very well with being merchants, in addition to their inherited lands and their husbands lands, most men would leave instructions to their wives on how much of their money to use in risks/expansions.

To me this also fits a lot better then putting them on the battlefield, which I don't think happened in a majority of the factions involved.

This is of course unless the princess character is hard coded to be a diplomat instead of merchant.