BTW it would be good to give all players a motivation to go through the tech tree. So it would be nice, if this GB would require promethium for building and it should be so good, that it is really wanted. At the moment there are almost more advantages in staying in a low era than advance to the end of the tech tree.
This is an interesting idea. Requiring Promethium/Orichalcum to build a GB would almost 'lock' it to AF and OF players only, which could allow it to provide rather higher bonuses since lower age players would be mostly unable to obtain these buildings. However, it's still possible for anyone to buy these goods with diamonds. Is it alright for the game as a whole for diamond players to have access to a building with Arctic or Oceanic-level stats while they may only be in the middle ages? They already have access to various better buildings if they so choose, but a Windmill is a far cry from having something balanced around players in the Future.
Another solution would be to have the majority of GB collections adjusted to the player's age. While this would have no effect on the 'main' bonus for the Kraken in this case, if GB collections are largely based on both the age of the building and the fact that players are able to obtain all GBs at any age, I imagine this would mostly mean that GB collection amounts would increase which, usefulness of said collections notwithstanding, could be a positive change for players. Or at least, I'm almost certain that things like medal and supply collections on GBs consider the fact that a low-age player is able to have these buildings, which is why they seem so 'low'.
On the whole, I think that GBs in the higher ages are built to appeal to lower-age players as much as if not moreso than players in the age of the GB. It veers more towards the 'weaker' bonus in these instances. For example, imagine collecting this many medals per day from a GB when your medal rewards from other sources don't even number in the hundreds! This applies to other collections from GBs as well, and, again, may be why these numbers are so low and unappealing to higher-age players: They're designed to entice lower-ages into somehow acquiring the goods to build these GBs to obtain collection bonuses that are miles ahead of what they're able to produce on their own. That's just how I'm looking at it, though!