NOT COMPATIBLE with saves from previous versions (more information).

TLDR: Changes to Diplomacy and Security.  Romance has been improved.  Several story events have been added, and many fixes and smaller changes.  Story mode has been disabled.

Diplomacy

Diplomacy has been re-balanced so that the passive effect of improving your house's diplomacy score is weaker, and sending the right ambassadors to specific leaders is more important (and more effective). 

• You can now swap an ambassador, with the foreign head's permission
• Ambassadors are now more effective at diplomacy on people that like them, and vice versa
• The "House Diplomacy Efforts" opinion is now less strong
• Ambassadors now have a more positive effect on opinion towards you
• Ambassadors sent to a leader or archon will now also influence (to a lesser extent) the opinion of that leader or archon's vassals towards you
• Ambassadors will now be recalled when a ruling house falls from power
• The "Send Ambassador" action no longer appears in the context action list of dead factions

Security

You can now assign security team members to investigations.  This gives you both a chance to discover the secrets of a particular character, and gradually reveals more information to you about the subject under investigation.  You can now also cover up secrets.  This makes intrigue actions (such as murder / romance) much more viable.

House Management

• The fair population limit of a house now grows more slowly in proportion to income
• House heads of small houses now more likely to expand their house
• Reduced the amount of times rulers and leaders fire and replace their council members
• Characters no longer ask to be promoted to the council when it is clear you would refuse
• Rulers and leaders handle promotion requests more intelligently
• The opinion of a ruler to their leader from taxation is now updated correctly in scenarios where an old leader is re-instated (such as when they temporarily fell ill)
• Adopt Member action now shows moral consequences if your house is too large
• The "Effects" section of the House Score tooltip is now visible for other houses

Romance

Romance has been tweaked so that it is less wooden.  Hopefully characters will fall in and out of love more believably, particularly the player character.  Romance events probably still happen too frequently and will need further balancing.

• You are no longer automatically aware that you have become the love interest of another character
• You can only propose a secret intimate relationship to the character you love, or to a character you know loves you
• Characters now get jealous when their love interest's love interest or secret intimate relationship is revealed
• When two characters who are secretly in love are married, their love becomes public
• A character who is made a romantic proposal by their love interest is now likelier to accept
• When your security team discovers that someone unimportant is in love with someone unimportant, they do not bring it to your attention as a popup
• A set of romance-themed story events have been added

Politics

• Wars were mistakenly being ended "after a long period without hostilities", despite attacks having happened
• When a leader submits to an archon, other leaders at war with the surrendering leader now get visibility of the archon
• When appointing a new ruler, an archon can choose nobles that are in any faction in their league
• AI Behaviour
  • Rulers and leaders with the Loyal trait now less likely to rebel
  • Rulers and leaders less likely to fine a vassal if they are not desperate for cash
  • Aggressive rulers/leaders now more likely to resist being deposed
  • Leaders that submit to vassalage or surrender are now less likely to immediately betray their new master
  • Leaders are now less likely to demand/request submission of a potential vassal that hates them (since they cannot be trusted not to rebel)
  • Leaders are now better at identifying the factions they want to attack or ally with
  • Vassal leaders now pay more attention to the crime of rebelling against an archon
  • AI less likely to attack if it's out of AP

Claims

• The claim panels now have an info icon that shows who has championed the claim in the past when moused over, and how long till the claim expires when relevant
• Claim state tooltips now show when the claimant will acquire a stronger claim (if relevant)
• A new status has been added that shows the claims a character stands to inherit
• A hint has been added when you can press or champion a claim to expand your empire
• A vassal leader can no longer press their legal archon's personal claim to lead another faction
• Archons can now champion claims against leaders whose factions do not border the archon's personal faction but still border the league
• When the defending faction conquers the disputed system in a war championing a claim, the game no longer acts as though the defending faction was championing the claim

Morality and Punishment

• A character can now punish or request justice against a character who committed a crime against the former's underage descendants
• When punishing a character for multiple crimes, the largest crimes are punished first
• When a character punishes a criminal that you requested justice against, the event will always be shown to you
• Some morality-related opinions were not being saved correctly
• Some charges between independent leaders and archons where being removed incorrectly

Game Start

• It is no longer possible to start the game without having at least six other factions in your vicinity
• It is no longer possible to start the game as an archon (starting positions are slightly weaker as a result)
• Slow Start now allows a player to offer vassalage from the beginning, since they may be necessary if they are under attack at the start of the game

Updates and History

The History Panels now auto-filter to the highlights for that entity, making them a useful way to remember the key events that happened to a particular character / etc.

• World state changes that do not generate updates (e.g. a change in a character's traits) are now reported in the description of the relevant update
• The History Panel for houses will now only show updates that happened to members of that house
• When an update involves a character in an unknown location, their portrait is no longer shown on the map (thanks Antonia)
• Tweaks to suppress some updates from popping up

UI

• Map modes are now sensitive to the entity that's focused on the left, allowing you to e.g. see the combat predictions for another faction
• Focusing on a character while the player is not also focused will show the opinions between that character and the player next to their portrait
• Mousing over a faction on the map will now bring up its tooltip
• Tooltips for unknown factions no longer show the faction's status symbols and heir / acting head
• Added tooltips when mousing over the map in all map modes
• Tweaks to the UI tutorial (thanks Zahari, Sarissofoi)
• Replaced various icons (thanks Alan)
• Resolved a series of UI crashes related to area panels not being automatically closed when an event made them invalid for the currently focused object
• The main menu Social Media icons now adapt correctly when the UI scale is changed (thanks Zahari)
• The All Actions list no longer creates duplicates of the Grant / Revoke child permission actions
• When opening an entity subpanel, list were sometimes being scrolled to an arbitrary position
• Added volume controls for music and sound effects

Miscellaneous

• Story Mode has been disabled 
• Losing the game now shows a stats screen for your lineage, and allows you to continue playing as a new leader
• Archons can now invest in the systems of their vassal factions
• Reduced memory consumption
• Added more hints
• Fixed an edge case where you you could not ask a vassal leader to help you in combat if their capital was disconnected from your league
• Emotions and options were sometimes being incorrectly modified by the Honorable / Rule-breaker traits
• Various story events have been added

Thanks to everyone who uploaded a crash report when the game misbehaved.