Taken from the live letter official translation. Better details here than what I have above.
* The following is an overview from the information presented during the show.
The Forbidden Land of Eureka is an unexplored, untamed wild where the very elements are constantly in flux. While exploring Eureka and growing in strength, players will have the opportunity to obtain and enhance Eureka gear and weapons.
Strengthening the weapons from Eureka isn't required to keep exploring.
Up to 144 players can occupy the instance, so think of this as a new field area.
Players may form and disband parties with other players at will.
Player Progression
Players will utilize a new feature known as the Magia Melder.
Eureka will continue to expand during the 4.x series, and the level cap for the elemental levels will also increase.
Elemental EXP
Elemental EXP is experience points that can only be earned inside Eureka.
The more players you have in your group, the more that the EXP you earn will decrease; however, in exchange you'll be able to get chain bonus. You’re also able to earn experience points solo as well.
The Magia Board and Magicite
Think of it as adding elemental properties to yourself for battle.
The Elemental Aspects of Enemies
Enemies encountered in the Forbidden Land, Eureka are imbued with powers aspected to a particular element.
By strengthening the thunder element, your damage against water-based enemies will increase, and by strengthening the water element, you’ll have a greater defense against water-based attacks.
Penalties incurred when KO’d
When players are KO’d and their time runs out causing them to be warped back, they will lose elemental EXP. However, if they are revived by another player before this, they will not suffer the penalty.
(We showcased gameplay of The Forbidden Land of Eureka on the development server.)
In order to enter Eureka, you must talk to the NPC "Rodney" in Kugane.
You can also check the Magia board here.
There is also an Aetheryte in Eureka, and the whole place is a unique field area.
You'll also encounter familiar NPCs such as Gerolt and Krile. We hope you're looking forward to seeing why they're actually there.
There are repair shops and shops that sells consumable items that are useful within the area as well.
Although it was sunny in Eureka during the preview, the weather is often bad due to the existence of untamed wild elementals.
You also cannot mount up until the exploration is completed.
The attributes of monsters can be confirmed by targeting them. Basically you should choose the advantageous attributes for yourself when comparing the attributes of the enemy. You can select attributes by rotating the Magia board for a predetermined number of times. There is a cost required to rotate the board, and it will recover over time. You can also recover this cost by touching the Magia Melder located at base. In addition, the Magia Melder can adjust how many Magicites are assigned to each attribute. Depending on your plans, you can choose whether to assign a certain attribute heavily or assign them all evenly.
Monsters in Eureka will follow you for a much longer distance compared to the regular monsters that you encounter outside Eureka. Since you will receive a large amount of damage from enemies whose elemental level is higher than yourself, it is quite difficult to shake them off.
Notorious monsters may appear under certain conditions and these monsters will drop rare items. The contribution to the fight works the same way as FATEs so it doesn’t mean you must land your attack before others.
Player search inside Eureka is also available. Since it is possible turn on a “looking for party” status, you can use it to group up with players with similar elemental levels as yourself on the spot.
Rewards in the Forbidden Land, Eureka
There are Eureka gear and weapons.
While exploring Eureka, players may obtain protean crystals. These crystals may be used to obtain and enhance Eureka gear and weapons gradually with the aid of Gerolt.
We’re planning to change the play style as we introduce later versions of Eureka.
Players can use the player search feature to search players within Eureka.
By changing the players online status to seeking party, players close to your level will be displayed, and you can invite them to parties. You may also shout to form parties, use the player search feature, or proactively form parties to play this content.
Players will receive AF3 (job-specific equipment) weapons with special effects, and gear that are dyeable as a reward. We also have gear which we received requests to remove role restrictions from.
We initially pushed back the release of Eureka to Patch 4.25 so it can be played once everyone has nearly completed gathering their token gear and defeated the raids.