Minoan Crete (playable once reaching Iron Age)
Why?
Many historians believe that the Minoan culture could likely be linked to the myths of Atlantis, so it fills in an aspect of that very popular idea for a culture.
The Minoans
were known to be more highly advanced than the Greek & Egyptian empires of the time, having built things like working aquaducts long before the Romans did.
Their look is quite unique being a blend of Greek + Egyptian styles in many ways.
Reward Buildings:
a) A large, (4x4), temple that gives a large atk+def% boosts to the
defending army, plus daily fps + guild goods + random military units upon motivation. (atk+def% boost of 30% similar to the Yggy Tree from Vikings at the very minimum!)
b) A smaller, (2x2 or even 3x2/2x3) monument that gives fps + goods + a small (ie: 3-5%) atk boost to the
Attacking army.
Mini-Game:
St. Pattys event style mini-game where you must move your cultural currency between different 'trading posts' along the canal system, with the various tasks, (ie: upgrading merchant managers), rewarding an amount of 'offerings' which are used to then build cultural goods.
Doesn't have to be as intensive as the St. Patty's event in terms of the 'hands-on' nature, but instead of of having 3 lists of recycling tasks, just a long single list of tasks that will provide more than sufficient offerings to build all require guild goods...
...also, the cultural currency acquired via the mini-game is how you build additional settlement buildings. (ie: residential, culture, goods)
now I know the idea of a huge boost to the Defending Army is likely going to elicit a lot of hate, BUT!, we do have the re-working of the PvP Tower on the very near horrizon, and this fills in a big niche without gobbling up a stupid amount of space within a player's city.
Plus, the main value of the building would still be entirely within a substantial daily fp collection, (ie: minimum 8-10fps/day, though 12-14 is ideal), AND, a large amount of Guild Goods, (ie: 50-75/each good), & at least 4-6 random military units thus makes it a very inviting addition for anyone in an active GBG/GvG guild.
We also have plenty of sources for boosting the Attacking Army already, but boosting anything to do with the Defensive army for the most part is a total & complete waste of space at this time, since almost all of those buildings with *do* boost the defenders stats have 0 additional benefits. (ie: only a couple event buildings like the Hippodrome ever get built, since it's main function is focused on fps/goods/etc...)
Thus, a Cultural Settlement building that still requires a sizable amount of effort & time investment, yet which still focuses mainly on fps/guild goods/units won't 'break the game' in any way.