It was mentioned in the Eureka quests that the region they've dubbed "Eureka" is actually the former Island of Val, which holds the Baldesian Arsennel and the Primal entity known as "Eureka". However, it was also mentioned that Eureka was named as such as an homage to the legend of of the Allagan stockpile of legendary weapons under the Syrcus Tower.
Playing on this further, any astute FF3 fan would know that at no point in Anemos/Pyros/Pagos/Hydatos does the actual themesong of "Forbidden Land, Eureka" from FF3 play, save for the first few notes when entering a NM FATE or when an NM FATE is won. Moreover, the overworld theme that plays when in a Eureka instance is simply a remix of the normal FATE theme (listen @24seconds).
Do you guys think we may get Eureka as the relic quest in 5.0/public dungeon, or will it be the 71 MSQ dungeon? The 5.0's PotD/HoH? Personally, since we are returning to the Crystal Tower before entering The First, I can see the level 71 area either being Eureka beneath the Crystal Tower, or it being our first dungeon that takes us to The First.