One of the bigger ones: Detachments as you knew them will be gone. With 8th ed, it's all about steadfast for infantry, and since steadfast is not mitigated by disruption, Detachments does not really make much of a difference besides giving the enemy more attacks back at you. On top of that, you need a minimum of 10 models to even cause disruption, hence any detachment would need to be 15+ models, almost a unit in themselves. Furthermore, since Steadfast is based on the highest number of ranks in a unit, you are usually better off just adding the detachment models to the main unit instead.
In my planned update, all regimental units can act as detachments if within 3" of each other. Meaning that a full unit of halberdiers can flank charge an enemy unit charging a full unit of swordsmen for instance, and handgunners and crossbowmen can stand and shoot against enemies charging any regimental unit, just not their parent unit. This encourages multiple smaller units rather than just hoarding up on halberdiers, and puts the focus on the Empire's infantry coordinating with each other more, as well as holding a coherent battleline, just like the Empire should. Enemies will now also have to plan more carefully, and must attempt to charge with multiple units to avoid being flanked.
As for other planned changes:
- Grand Masters will be able to ride Demigryphs.
- Spearmen will be able to upgrade to pikes (finally!).
- Free Company will have pistols as an option, and can upgrade to Ambush.
- Archers won't be skirmishers by default, and can have LA.
- Handgunners and Crossbowmen can have LA (allows upgrade to Ironsides).
- Knights will be kept core, but can be upgraded to one of 3 knightly orders (Panther, White Wolves, Blazing Sun). For the remaining Knightly Orders, consult the Expansion.
- Inner Circle Knights counts as special units.
- Hold the Line increased range for generals.
- Demi-gryphs are moved to Rare, will be increased in cost.
- Imperial Dwarfs added, spears, GW's, HA options.
- Halflings added. Spears, bows, shields.
- Reiksguard Foot Knights added, Reiksguard Knights will stay as normal.
- Flagellants nerfed to WS2, can sacrifice D3 models each close combat phase.
- War Wagon added.
- Steam Tank variants added.
- Cheaper state troops, miltia, greatswords, huntsmen, pistoliers, outriders and flagellants.
- Mortar buffed to S3, slight price increase.
- Helstorm buffed to S4.
- Helblaster firing mechanics tweaked, can fire single decks.
- Extended fluff in both the main background and for all units.
- Return of old special characters, as well as the ones from Tamurkhan.
- Imperial Ogres added.
- Arcane Battle Altars will allow upgrades to all different colleges, thanks to rules by Thomas Heasman-Hunt.