Europa Universalis 3: Heir to the Throne builds upon the seemingly unimprovable In Nomine and not only does it better the previous installment... it actually overhauls the entire game!
- Papacy: Reworked the Papacy Screen and mechanism. No longer can we bribe our way to the top, we now have a variable called "Papal Influence" which is affected by our NIs and Sliders... which in turn directly affects our "Cardinal Chance" ... While this more closely resembles real life, it somewhat lacks the struggle spirit the previous model had, being one now pretty powerless to intervene.
- HRE: The HRE is built from the ground up... reworked from scratch! Now the Emperor can enact Imperial Reforms which increase his power over the Empire... to do this he must spend Imperial Influence, which he gan get by doing various actions that aid the Empire's overall welfare.
- Casus Belli System: A new, overhauled Casus Belli System makes it way harder to go on killing sprees all over Europe... You need a proper CB to wage war on a Country, and different CBs mean for different bonuses (say, Reconquest allows for cheap annexations and demanding of territories in which one has got a core, while attacking because of an Insult allows for cheap war indemnities and concessions).
-Spheres Of Influence: Arguably the least useful of the expansion, but worth mentioning nonetheless: This new diplomatic action allows you to project your influence over other countries. Now, this move gives you an Intervention CB on anyone who meddles with any country under your sphere... but that is pretty much it... It does not really add to the overall gameplay, and one can skip it altogether.
-Trade Leagues: Reworked the EU III Market! Now one can choose to join a Trade League (or lead one, for that matter), conceding trading rights in exchange for production rate. It does certainly spice up the Merchant Republics' gameplay.
-Magistrates: Magistrates join the ranks of Spies, Diplomats, etc. as they are the ones who let you enact province decisions and the brand-new Cultural Decisions. These, in turn, may gain you Cultural Tradition, which leads us directly to...
-Improved Advisor system: Tired of having to scour the world for that particular advisor?!? Look no further! With enough Cultural Tradition you can get whatever Advisor you want. As with Military and Naval Tradition, having more means there's a higher chance a good advisor appears.
-Dynasties: Finally Dynasties make it to EU III! However, much to my dismay there is no real advantage of having all of Europe ruled by your cousins... just some random events here and there... but it does improve the overall feeling of the game...
-Successions & Thrones: Well, this part received a massive overhaul... Departing from In Nomine's setup, in which successions and Inheritances were random events you can now actually know which country is likely to be inherited or get into a PU and with who. Moreover, you can now try and claim another country's throne and either wait until the ruler dies (in which case it may very well usher in a War of Succession between two claimants) or wage a war on the target country for its own throne. Beware, though, for countries with which you have Royal Marriages take not very kindly the idea of you expanding at their own expense, and claiming a throne will destroy your relations with other countries.
Finally, a History function was added, so that you may check your country's history without having to exit to the main menu.
Overall, this add-on is a must! The vastly improved HRE, the overhauled Inheritance model, and the built from scratch Casus Belli system are more than enough to justify your purchase. Add to this all the other features and this is a no brainer.