Not sure if this has been posted yet, but I found it here on reddit:
City fight is a thing. It has special detachments, special rules and stratagems in the new campaign book.
There's a new army mechanic for picking detachments.
There are new "mega stratagems". They effect a whole load of units in a detachment. For example they can give every unit in a detachment a keyword.
GSC are in it, they get special detachments too.
The Orks have a stratagem to make a Stompa their warlord. There is even a Stompa warlord trait table.
These rules will be used in matched play.
You pay CP for specialist detachments - rather than in 7e, where formations were free.
They are more specialised benefits, so are quote-unquote "easier to balance".
An example of a stratagem/benefit is "indomitus veterans", which upgrades all intercessors in a detachment to veterans.
They said they were aware that armies with a load of CP (e.g. guard) have more CP than more elite armies.
The benefits only affect specific models (e.g. all manticores in a detachment).
Urban Combat is a box. There's a plastic wallet inside with cards to make a map (for building your own hive city). The overall map is "A1 A2 ish". You slot the cards together to make the city. There are apparently reusable stickers you can peel on and off to show control of a given area. The box also contains more content, which wasn't touched upon further.
The specialist detachments are more "characterful" rather than "powerful". These detachments won't be for every army or model out there - they'll just be for the armies and models that make the most sense.
These detachments are not intended to be a "replacement" of the 7e detachments.
Vigilus will be around for a while.
No Necrons and Tau on Vigilus. They may appear in later settings if they suit it. They don't want to make an unrealistic setting where every faction is present.
The setting isn't just one environment (e.g. everything being a frozen tundra). There are different biomes and a load of variety. Paul Denton is the artist who does the environmental pictures for it.
Water is super precious on vigilus if you didn't know this already, admech use a space elevator to fetch water.
Some guard detachments include one for Russes, a Tempestus Scion one, a Leman Russ one, a Sentinel one.
The factions present on Vigilus are going to be getting these rules.
Notably rather than "AdMech" being listed as a faction, they were instead listed as "Kastellan Robots". Eldar were also listed as "Windrider Host". All other factions were their general name (e.g. "Orks", "Genestealer Cults", "Space Wolves").
Some armies won't be appearing in campaign books (if they don't fit with any of the planned campaigns).
Normal space marines can be made into primaris now with surgery (or something).
Again, mini-marines aren't going anywhere.
GREY KNIGHTS ARE NOT GETTING THE PRIMARIS TREATMENT.
As to whether they'll have a load of Astra Miliatarum datasheets printed in the GSC book? No.
However, anything you've been able to ally in previously to GSC using the index is possible now. They used manticores as an example. Don't want to make previous purchases wasted.
They have tried to make them stronger as a standalone army - said that many GSC lists were just a tiny amount of GSC, and then a load of Tyranids and/or IG.
Strongly suggested they'd be getting subfactions.
The codex adds a load of options to the army, it's almost a complete replacement (said in relation to subfaction keywords especially).
The codex has exciting new mechanics, stratagems especially.
The releases shown today aren't the full set of releases - there is more to come.
Cult Ambush is getting something turn 1 - not the full deep strike like it was pre-FAQ, but "something more" than just a 9" pre-game movement (a la Alpha Legion/Stygies/Raven Guard).
Renegade guard codex isn't "in the works" right now.
There is more than one campaign book in the style of vigilus coming.
Each setting has its own theme.
Imperium will be in each campaign setting.
The next campaign setting is very fleshed out already - similar in structure to what we see with Vigilus and City fight.
Campaign books like city fight won't be limited (i.e. there won't be a 6 month window in which you can buy them).
There are more codexes coming out next year.
There are currently no plans to release an "agents of the imperium" codex. The designers felt that the units don't translate to the battlefield well. An inquisitor etc. feel more appropriate for rogue trader/kill team. Specifically gave example of a character with a simple rapier and a laspistol doesn't feel "right". They actually used Eisenhorn as a very specific example too.
For boxed games they usually work about a year in advance.
The person giving the seminar was working three years ahead on some stuff. Miniatures team plan significantly further than 1 year.
More rogue trader explorations (or at least this style of release) will come in the future.
Beta codexes are a sort of thing we'll be seeing in the future, but not in the same way as the Sisters of Battle beta. The beta rules will be put in white dwarf (e.g. a datasheet, a few stratagems), if they're well liked they'll find their way into matched play 40k (maybe with tweaks depending on feedback).
Sisters of Battle should be good standalone.
They don't want to punish soup. They'd rather reward builds than penalise them. No comment on whether they'd try to reward monofaction armies.
They are aware that they don't quite "get" soup yet.
Points are worked out using a formula, some of which depend on points themselves.
Open play rules are mainly to "scratch a certain itch", and gives an opportunity to add more "creative and crazy" rules into the game without breaking play.
They know the fly FAQ change was an issue. They wanted to solve a simple problem which lead to oversights. They want to create simple fixes rather than add a load of complication to the game.
They don't want to be changing the rules constantly and frequently. They say that most players aren't in the tournament circuit. Instead missions included in Chapter Approved etc. are meant to put a new spin on the rules.
They don't like the idea of having too many required books either (e.g. having one page from a campaign book, two from white dwarf etc) - they say that it will be possible to build an army without needing hundreds of different sources.
They say their goal is to have it be possible to make a good army with fewer sources. They used GSC as a specific example.
Index options are here to stay for a while. They're just not in the codex because it's bad to have a codex with selections you can't buy.
Kill team has more coming.
The miniatures team make the concepts for models, and when it's fleshed out and the model is designed they send it to the rules team to make rules for.
Miniatures team always comes first, design team doesn't really present to miniatures team USUALLY. So no "orks need a flying thing, make a model for that".
Since the rules designers work maybe a year or so in advance, they sometimes get confused on rules (because they're not going to be implemented for a few months/a year).
Robin Cruddace will have a column in future White Dwarfs to explain ways you can play etc.
The codex design team is lead by Robin Cruddace and the seminar person (can't remember name). Also on the team are James Gallagher, and "a few new guys".
They won't design primarch like models for each army (e.g. nids, tau, orks). They'll just keep making cool models, if it happens to be Lord of War size then sure, they'll get a lord of war.
Primarchs will be done rarely, to keep the feel that they're "special".
"We should totally do Angron soon!" was a direct quote by the seminar person.
Another direct quote is "Sanguinius looks pretty dead to me."
Whichever primarch mini they want to make next is whichever one they want to make (coolness).
Robin Cruddace did a PhD in Physics and apparently really loved maths. (This actually seems funny given the internet memes of him being the guy who said "maths is just an opinion" etc.).
GW do keep an "ear to the ground" on online forums (indirectly and directly). They just don't really post. It's hard to take suggestions from online because of how many good and bad ones there are. The Ork dakkadakkadakka rule was actually thought up by the design team before it was also suggested online.