Research costs

[Seravy]

By 4 but yes, I guess that's one way to look at it.
Old : 12/24 = 50% bonus. New : 42% bonus. That's still a relevant difference albeit not that bad. Especially as the relevance of the bonus in the early game is now less. Previously, playing Elf was the one way to have acceptable early power. Now, the few extra points you can get this way won't even substitute for a single new magic building.

Halflings are currently 2/3, so raising them to 1 would make sense.

[Seravy]

Question, how should the AI treat the new Magic Market and Library?

Library was already considered a "basic building" meaning it'll be picked over "nonbasic" buildings 99% of the time, but the mandatory choices still came first (Smithy, Markeplace if critically low on gold, whatever military was required, ship building if that was required by the map type, settler) and if the AI picked "I want a unit" then it wasn't picked.
The new building is new so has no rules obviously, for my first test game I set it up as a basic building like the library but it had a typo so now I have to start over the game as the AI was instead set to never building it.

Question is, which rule do we want for each of Library and Magic Market?
1. basic building (this basically mean they pick randomly from Magic Market, Marketplace, Library in no specific order but will usually get them first. Expert+ also includes Granary here.)
2. mandatory building (before military building? after? after settler? Or before even the smithy and 4 swordsmen?)
3. mandatory building but conditional (for example if low on mana, or difficulty above a certain level)

As this test game with a bug showed, if the AI does not build them, they'll be massively disadvantaged, I was able to overwhelm two of them with my ghoul stack as they didn't have the usual massive summon armies. (difficulty was Advanced and I reduced the AI power advantage to match gold as an experiment, this might not stay if it doesn't work but we'll need time to tell...I also had a good start on my side.)