Buildings:
I really like the diversity between the structures of the different playable factions. Especially the Greeks, Romans, Carthagians and the Celts feel good in this respect. The Iberic tribes, Nomads & Germanic tribes could use abit of love. One of the problems I had with the Germanic tribes was getting my hands on Drugs and Medicine for the Irminsul Gathering building. The closest place to find it is in Britain, but the tribes there usually do not have the trade range so the only real option is conquering Britania Inferior (unless I missed something).
One thing that bugs me abit is that it is very difficult to get the right militairy buildings in capital cities of provinces. Perhaps making them province wide and unique to the province or alternatively if the event that lets you focus on different structures (harbors, workshops emporiums, defensive structures) would also have an option for structures like the baracks, stables, etc. It would also be nice if units built within a province that has a master fletcher/master armor smith would come with the defense/attack related upgrades.
Manpower: Feels like its too easy to get. Tribal Nations and Rome pretty much never run out. With nations like Sparta I don't have issues with it anymore after turn 60-70, Carthage takes abit longer. It's also a shame how early mercenary units get rendered obsolete considering they are abit mediocre.
Raiding: Feels underwhelming, you rarely get slaves when raiding and the monotary rewards are quite small. I think there is an opportunity here to make tribes alot more interesting. Giving tribes the ability to move into neutral territory to raid.
Sacking Cities: Adding an option to sack cities different culture to get more slaves and legacy or to reduce decadence would make the world feel more dynamic (cities only get bigger and they do not decline).
Money:
Commerce bonusses feel abit lackluster because it is so easy to get money in the game, unless you are tribal where it is atleast an issue in the beginning. I also never employ citizens on commerce jobs.
Culture:
Really (too?) hard if you are tribal on higher difficulty levels, also relatively hard as Carthage (also feel Carthage is abit of a pushover in general). Abit too easy when Greek (although anything is generally easy as a Greek except dealing with Rome

Random events:
Would be nice to get a few more random events, harsher plagues, vulcanic eruptions, invasions of tribal hordes (Huns?).
Cavalry:
I rarely see an argument to putting any medium/heavy cavalry in armies. If the combat width is filled, they do not really fill a role other then chasing down enemies and this can be done just as well with light Cavalry.
One solution could be to lower their attack/defense stats by 1/1(so they are worse in hills, arid hills, mountains, forests) and giving all cavalry a +2 attack/defense bonus in plains, deserts, arid terrain and steppes (perhaps removing some of the bonusses they have now).
Cataphracts are arguably better having the same attack value as Roman legions with less effectivity and hits (only surpassing it by +1 one if you manage to get the stable master).
Bugs & general feedback:
Should I be able to make irrigation channels in Germany (for Papyrus)? Cattle and to a lesser extent Sheep in the desert? Was also suprised to see Hemp there (even though it uses little water)
It would be cool to have a building browser in the game. Sometimes I have a hard time to figure out what I can build where. For example, I know now you cannot build T2 Farms outside plains, but it doesn't say anywhere (as far as I have seen).