Fantasy Flight Games has done an outstanding job with their recent remake of the Horus Hersey. Hats of to Jeff Tidball, it is an outstanding game. That said, the game left out some key elements that would make an exciting expansion. They are as follows:
1. The Sons of Horus Space Marine Legion is missing!! The most climactic battle of the Hersey and the leader of the rebellions own chapter is AWOL?
Recommend adding 4x Space Marines and 2x Thunderhawks to correct the error.
2. Imperial Thunderhawks. Although the majority of Imperial air assets were lost in a likely space battle with the Chaos Armada before the planet fall,
it is likely some assets survived. To reflect this, recommend you give the Imperial player only 3x Thunderhawk units, 1 per Legion vs. 2 they would likely have had to represent the likely early losses. Note: Imperial Thunderhawks would also balance out the addition of the Sons of Horus. The Imperial will also need some new combat cards similar to the Chaos "Rain of Death" to get full benfit of the Thunderhawks, but likely less (say 1 or 2)
3. The Sky Fortress would make an impressive unit and further balance the game with the addition of the Sons of Horus Legion. Recommend the Sky Fortress be a Rank III unit, Move 4-flying which can bombard, transport-3, always treated as fortified, armed with a Defense Laser, and can only be attacked by Thunderhawks, Space Marines, Titans, Traitor Armor Divisions, and Dameons (units that can fly or likely have anti-aircraft capability). You would have to replace two pro-imperial event cards though.
4. Add Thunderbolt Fighters Imperial Navy aircraft assets are also missing, and likely also attrited by the initial space battle. However, additional
units can be built and they can be corrupted. Recommend: Mark I, Move 5-Flyer. Can "intercept" flyers (see AA rules below). I would allow the Imperial Player to start with 2, with no more then 4 more in his force pool. You would also have to add Imperial order cards to build new units.
5. Defense Lasers: Expand Defense Lasers role so they have a function beyond the drop pods and port landdings a. Allow them to also represent AA fixed sites (see AA below).
b. Allow them to represent fixed Deathstrike Missile sites, an Event card, allowing the Imperial player to bombard an area within 3 areas of a defense
6. Anti-aircraft combat: Add a combat 0 iteration round for anti-aircraft fire vs flyers. Only select unit can fire and only get 1 shot each. Add all AA units firing (both sides), and both draw a bombardment card (attacker 1st), look at Defense laser section under the number of AA units used (max 4). Remove the number of hit "Flyer" units only. Then proceed with battle as normal. AA units are as follows:
a. Defense Lasers (actually represents static small hydras, sabers, etc) defending the Defense Laser and the area.
b. Thunderbolts (and eventually other fighter aircraft)
c. Certain select units by new combat cards (Armor divisions - flak Hydra or Manticore weapons, etc) Note: AA rules allow Defense Laser a more enduring role in the game as well as Thunderbolts and give a check and balance to prevent Thunderhawks from running totally amuck.
7. Titans and Armor Divisions can conduct a distint strike attacks using the Thunderhawk portion of the bombardment card to dertemine effect. Attack represents these units Deathstrike Missiles, heavy artillery, marco-cannons, etc found in these unit types. When promted by a new combat card (and possibly an option for a Titan Breach card) owner draws 1 bombardment card, uses the Thunderhawk section, strength of attack equal to half (50%) of the titans / Armor Divisions current rank, rounded up. Note: This gives a little more punch to these units and represents the effect of Imperial artillery.
8. Unit Discs for Titan Legions. For Imperial: The way the game identifies the Space Marine Legions with small disc is an outstanding, simple, nice touch. However, the Titan Legion, also were distinct fighting units with a pround history. This small addition would add more to the game by smily including them in an expansion. For the Imperial player it is the Fire Wasps, Templars, and War Griffins. For Traitors it Storm Lords (Slaanesh), Flaming Skulls (Tzeentch), Deaths Head (Nurgle), and Khrone symbol or Chaos Titian Eye symbol for Khrone as these were really the Khrone War Engines and not a Legion per say.
I have been play testing all the above with positive results.
My 3 cents ...