A must update to the original
Heir to the Throne is probably the last major update to Europa Universalis III, however it looks as if Paradox has saved the most complete overhaul of the gameplay mechanics for the last. The myriad of changes and polish applied to the game, and still ongoing via various beta patches, continue to evolve the gameplay formula to heights never seen in a strategy title before. All in all, this is a must update to the game, and makes the game more engrossing, if even that was possible in the first place.
Coming from the Napoleon's Ambition, and skipping the In Nomine expansion, particularly because I was rather miffed about Paradox releasing what seems glorified patches as PC's equivalent of DLCs, nickle and diming gamers, I was at first apprehensive about the breadth of changes and additions that would have been applied to the game. The core gameplay of the original I found pretty decent, yet vulnerable to exploits and tedious micromanagement in the later stages of the games.
In the end, my worries were unfounded. The updates were very thoughtful, surely due to the developer maintaining a responsive relationship with their loyal fanbase, and listening to their suggestions. I am sure you could peruse the bullet points of the new features on the update's feature list, but the impact of those enhancements are very welcome.
Among the most notable features that I can list, the historical nature of EUII in terms of monarchs and other prominent figures was incorporated, as per its new historical setting, and allows alternative aspect of keeping historical prominence of certain nations without resorting to setting random 'lucky' nations. Rebels now have been differentiated and have a specific cause and size tied to their uprising, such as religion or ethnicity based. Religious conversion and colonies are now funded per monthly income flow, and can be variably set to affect their outcome. The armies now have indicators suggesting an ideal combination of calvary and infantry, which makes this aspect of the game less morbidly confounding and accessible to novice players. The most important aspect I noticed was the ability to be involved in a conflict to determine the succession of a throne (hence the title), complete with hereditary claim and royal marriages. And owing to expanded list of casus belli, there are many situations that can embroil huge nations into war (stretching from Europe to the colonial holdings in Europe), something that was rare in Napoleon's Ambition.
All these additions are accompanied by tweaks to the gameplay itself, many of which are well founded and quite frankly surprising in how they render the gameplay with changes refreshing, but no so drastic as to require a lot of adjustment to your playing style. For example, calvary now does not have speed advantage to other units in the game, the nations at war offer more sensible terms for peace and not quite predictable.
All in all, this update is a very important one to the EUIII players, almost a no-brainer. It brings the unique EUIII gameplay mechanic to a another level of polish. Of course, the late games do get a bit mired and tend to stall a bit, but that could be due to my gameplay style and should not detract from the millions of possibilities that the game already provides, and I should be playing trading empires a bit more anyways.