Aztecs: Get Halberdiers
Aztecs are already super strong in Early Imperial Age, they don't need another boost. Halbs with Garland wars would be insane.
Britons: Get Heavy Scorpions and Get Hussar and/or Siege Ram and/or Bloodlines
IIRC Forgotten gave them Bloodlines. Getting any one of these would be fair enough, Britons aren't the best in Imperial Age. Bloodlines makes them a very strong pocket civ.
Chinese: Get Paladin and/or Siege Engineers and/or HC/BC and/or Block Printing and/or Parthian Tactics
Paladin no, and any of the others wouldn't affect them much. Chinese are already powerful enough in Imperial Age, moreso I'd like to see a miniscule buff to them in dark/feudal.
Franks: Get Bloodlines and Squires
Bloodlines no, and I shouldn't have to explain why. Squires is a good idea, Throwing Axemen aren't very good and would improve their Halbs.
Goths: Get Plate Mail Armour and/or Parthian Tactics
Definitely not to Plate Mail, they are already strong enough in Imperial Age. Parthian Tactics would change absolutely nothing. Like Chinese, a tiny buff in Dark/Feudal would be more appropriate.
Huns: Get Ring Archer Armour
No, no, no. Huns are already super strong throughout the game until later Imperial Age. While this would improve that, it would moreso just stack their odds in early Imperial Age, Huns are dominate enough, they do not need a buff.
Japanese: Get Bloodlines and/or Siege Ram and/or Bombard Cannon
I'm less sure of this but I thought Japs get Bloodlines in Forgotten as well? no? Anyways any of these are fine, but Bloodlines would be the largest buff, like Britons would lead them to be a stronger pocket civ. I believe that their siege is lacking because their Trebs are supposed to be dominant.
Koreans: Get Bloodlines and/or Parthian Tactics
Anything that improves Korean's late Imperial game is a terrible idea. They need buffs from dark to castle, their late Imperial is overall the strongest in the game.
Mayans: Get Redemption and/or Siege Engineers
I can't really comment on this since they wouldn't change much, Siege Engineers would just make Mayans very slightly better in Imperial Age, which I don't believe they need due to their strength from dark to early Imperial.
Mongols: Get Plate Barding Armour and/or Ring Archer Armour
This is your worst suggestion. Their Hussars are fine, sacrificed arrow protection in favor of life total. Ring Archer Armor would make their Mangudai laughably overpowered (as if they aren't already) Mongols already have the best overall map pool strength in the game, with their only weakness being Castle Age. They do not need buffs.
Persians: Get Bracer and/or Siege Engineers Bracer would be strong for flank/1v1s. I don't think Persians need it, they already have a good economy and strong Calvary. Siege Engineers wouldn't change much.
Saracens: Get a tweaked Guilds where their trade fee is -10% (sell 100 food/wood for 23 gold) and/or get Heavy Scorpions
Saracens are already strong enough in Late Imperial. What they need is a dark-castle age buff to be useful in other stages.
Spanish: Get Siege Engineers and/or Parthian Tactics or Heavy Scorpions
Siege Engineers or Heavy Scorpion would be fine, but aren't a big deal. Parthian tactics would make their Conqs too powerful in early Imperial. Would be good for later since Conqs are always swapped into Paladins then, but the consequence is that their Castle drop conq build would become more potent, and the early Imperial push would be very hard to stop (with an economic disadvantage due to conq raids).
Teutons: Get Bracer and/or Siege Rams
Like Persians I don't think they should get Bracer. I do agree with Siege Rams though. A thing I find funny is how noobs always quote "Teutonic Knights in Rams" even though Teutons don't even get Siege ones. However, personally I'd rather Teutons just get Husbandry. Their Paladins are too slow.
Turks: Get Paladin and/or Arbalest and/or Siege Engineers and/or Block Printing No to Paladin, no to Siege Engineers due to their BBC already having +2 range. Block Printing wouldn't change much. Arbalest would be fine, but I'd rather see them get a dark/feudal buff.
Vikings: Get Halberdiers and/or Bloodlines
Lot's of people say Vikings should get Halberdiers, but they don't realize just how strong the Viking economy is. Halbs coupled with the extra HP would be fine for later Imperial, but the problem is that it would make a Viking early Imperial push near unstoppable due to their economic prowess. Similarly, due to Viking's economy, if you give them Bloodlines they would become nearly, if not the strongest pocket civ until Imperial Age. They can already pump out more Knights than any other civ in Castle Age, only contested by Huns. The only way to buff the Vikings properly would be giving them a buff purely in late Imperial Age, something like a change to their Berserks to make them not just viable, but strong, or giving them Siege Onagers.