Europa Universalis III: Heir to the Throne
Европа III. Великие династии - Russian spelling
The focus now is the struggle in diplomacy and internal affairs. The new casus belli system understands the differences of the motivations of war and act accordingly. Each different casus belli has a different effect on peace resolutions as give more prestige, less infamy and requires lower score to receive demands legitimated by that casus belli. And, accordingly to the situation, they may aggravate your situation in a defeat too due the sheer loss of prestige.
The other focus is the succession, as now, if a king dies without a heir (or a too young one), the following regency period could be disastrous for the kingdom, also rulers with low legitimacy may face strong opposition from pretenders to the throne. Some other minors details were changed, like he election of the pope, and the power of the Holy Roman Empire emperor.