Here are my ideas for those four civs:
Celts: Instead of giving Celts a new UT, my idea is this - change the bonus to Furor Celtica back to 50%, but make the 20% faster firing siege bonus into a second UT instead, preferably a castle age UT. That this is a good way to nerf the Celts a bit - now they have to spend more resources to produce their super-siege, but they still have the super-siege end game.
Mongols: I agree with Barb, except for one thing - make the UT affect all cavalry archers, including the Mangudai; however, lower the Mangudai's attack from 8 to 7.
Persian: I sort of disagree with giving paladins +1 more pierce armor. They'll now be like fast moving elite huskarls. I mean, seeing how ES thought giving Franks 212 HP paladins were broken, I sort of see the same thing with 8 armor paladins. I think Persians need a UT that will help boost their potential in Arena maps, which they completely blow at due to poor castle drop, poor monk rush, and poor early imperial. A castle age tech that not only help them in Arena, but also, on other maps as well, to a slight degree. Hmmm... how about petards + 50% HP? They can now do castle age petard rush, and it'll also make up for their lack of siege engineers.
Saracens: Oh boy... so much I can think of. In a way I sort of see Saracens as suffering from Frank-syndrome - slow in dark and feudal age, worse than Franks in castle, but better than Franks in imperial; though still not as good as Teutons, Spanish, Koreans, Turks, or Persians. Franks got the 50% faster stables now, and 10% faster ETA's, so maybe give Saracens something to boost their siege. How about siege onagers +50% faster projectile speed (similar to the Spanish cannon galleon bonus)? It also makes opponents think twice before using archers to kill elite mamelukes (because they are super duper expensive).
Celts: Instead of giving Celts a new UT, my idea is this - change the bonus to Furor Celtica back to 50%, but make the 20% faster firing siege bonus into a second UT instead, preferably a castle age UT. That this is a good way to nerf the Celts a bit - now they have to spend more resources to produce their super-siege, but they still have the super-siege end game.
Mongols: I agree with Barb, except for one thing - make the UT affect all cavalry archers, including the Mangudai; however, lower the Mangudai's attack from 8 to 7.
Persian: I sort of disagree with giving paladins +1 more pierce armor. They'll now be like fast moving elite huskarls. I mean, seeing how ES thought giving Franks 212 HP paladins were broken, I sort of see the same thing with 8 armor paladins. I think Persians need a UT that will help boost their potential in Arena maps, which they completely blow at due to poor castle drop, poor monk rush, and poor early imperial. A castle age tech that not only help them in Arena, but also, on other maps as well, to a slight degree. Hmmm... how about petards + 50% HP? They can now do castle age petard rush, and it'll also make up for their lack of siege engineers.
Saracens: Oh boy... so much I can think of. In a way I sort of see Saracens as suffering from Frank-syndrome - slow in dark and feudal age, worse than Franks in castle, but better than Franks in imperial; though still not as good as Teutons, Spanish, Koreans, Turks, or Persians. Franks got the 50% faster stables now, and 10% faster ETA's, so maybe give Saracens something to boost their siege. How about siege onagers +50% faster projectile speed (similar to the Spanish cannon galleon bonus)? It also makes opponents think twice before using archers to kill elite mamelukes (because they are super duper expensive).