We've had a lot of posts from
sponge, an id employee with a ton of great info. Here are some interesting bits from he and some others:
1. they are actively looking for content for their add-ons with some parameters
ID employee said:
i'm starting to look around for stuff that's newer, popular, doesn't have assets from other games, and the authors are small in number and still contactble. i tend to have a recency bias since the new stuff is usually way cooler than the 90s stuff, and people are way less likely to be familiar with the modern community output
contactble: fewer people to track down for legal docs
2. don't expect AD any time soon and not in it's current form (way too thorny due to use of Hexen 2 and other assets, team size and see #1 re: legal) you are free to load it in the engine as an add-on but expect issues if and until some tweaks are made to the engine