I really hope that the current monarchs family connections would play a big role in diplomacy. Look at the sort of fighting caused by the family issues surrounding the spanish throne (The War of Spanish Succession).

And then there's the ties between the Dutch and British thrones. England and the Dutch should be best pals, for as long as a Dutchman holds the British throne.

Then later on we have the attempts of Europe's royalty to rescue the French royal family during the revolution.

My hope is that we will have well developed, extensive family trees to track the various royal dynasties, and that politcal leanings and faction disposition should be affected by the various familial bonds/rivalries.

I would also hope we can use marriage as the diplomatic weapon it should have been in Medieval 2, and was in the original Medieval. Since your royal sons and daughters are unlikely to be a unit in battle or an agent (princesses) anymore it would be nice if we could use them as political bargaining chips.

That would also open up the chance of making claims to foreign territory if a king dies without legitimate heirs.