Austria - Diplomatic Marriage - absorb CS's
Could be interesting to assimilate CS's, depending on if you keep the Diplo vote afterwards, it could be a game-changer.
Byzantium - get a 6th belief for religion
I always loved the religion aspect in CIV, and with a nice boost (whether in early game or late game, I'm not sure) it could just leave me craving a religious victory.
Carthage - Phoenician Heritage - units can cross over mountains after 1st Great General spawn, but lose 50hp if end turn on mountain. Free harbors in coastal cities.
This is the one that drew my eye the most. Obviously reminiscent of
Hannibal's Invasion of Rome, this could make "impregnable" land features almost useless.
Celtic - More faith near forest
With the random Great Prophet feature, this could make them more likely to have the first religion. Although I would say this combined with Pictish faith-on-kill could be OP, this is probably one of many reasons there is no Religious Victory.
Ethiopia - units get more strength if enemies have more cities.
Perfect for stronghold/defense strategies, and since I usually play more culture/science focused, I may find this is my favorite in G&K.
Huns - Scourge of God - raze cities 2x faster. Immediate access to Animal Husbandry, +1 production for Pasture.
Seems kinda useless after the Ancient and Classical eras. Historical, but very under-powered in my opinion.
Maya - time/calendar related
Depending on how they make this "work", this could be very cool or very disappointing. Keeping my fingers crossed.
Netherlands - East India Company - luxuries can be traded but you still have access to its happiness.
Could just become the new Arabia, but oh well. No complaints!
Sweden - Nobel Prize - something diplomacy related involve major civs, city states, and great people.
Aah. I am much looking forward to finding out the UA for this one. I supported adding Sweden (when it was cool, of course, but still), and am very amped up to play as ol' Gustav kickin' some Ruskie @$$ in the Great War.