A lot of the suggestions i often see for AoC are short term problems or balance issues, likely a product of the server hopping that occurs (due to reasons that have been stated many times).
Yes Cleo is powerful. Yes spiders are strong. Yes carts are bugged and we all like the idea of more races and ranks.
But if we're talking big picture, systemic issues, try playing on a server past 40 days and tell me there is any content left to do. The fundamental systems of this game are great and a lot of fun but two of the major overarching goals - building a kickass army and taking territory - are so disincentivised in the 'late' game that you start to lose motivation to login. So to expand upon that:
Building an Army:
Why do you build an army? To fight other players with or to fight in PvE. PvE is an easy one to tackle, there is no PvE content after 40 days and little before that. After two weeks or less you can take out every world boss and after 40 days or less you can finish the tactic school. Done, no more PvE. Okay what about PvP? Let's divide it into duels and city capture. That big bad army you're so proud of that you've spend the last month building? A single bad duel and you've lost a third of it (something that can take upwards of a week or more to recover; a quarter or more of the play time you have invested so far) and the other 2/3rd's is in the hospital unusable for two or three days. Well there has to be some risk, right? We don't want some coddled game whereby there are no consequences to your actions. But the RISK has to match the REWARD. And it doesn't. At all. the reward to that risk i just mentioned is Honour, a currency of some use in the early game but, again, no use at all after 40 days when your honour has already been maxed out for a week or more.
Well what about City Capture? That IS the aim of the game, right? A perfect opportunity to flex your armies muscles and show what the last 40 days of quest grinding, hero leveling and army planning has got you. Territory is pivotal in a servers early stages but lets look again at the risk v reward of City Capture 40 days into a servers life.
Risk: The risk is the same as in a duel - an entire third of your army can be lost and the rest hospitalised for days, putting you out of action and setting you back big time. On top of this you may be assaulting walls which are, by this point, filled with golds and you're opening your base up to revenge attacks; you're risking not just your army but your entire resource bank.
Reward: A 60 hr quest timer and, lets say, a moderate increase in resource production (you may be fighting for a bigger city or you may just be fighting for some more resource tiles).
I don't want to fill this post with hypothetical math but i don't feel like it takes extensive calculation to realise that the resources invested to take a city are rarely, if ever, going to be made up as a consequence of taking a city once the server has reached its 'late' game, 40 day mark.
And ultimately what it comes down to is that this big army you're building, this castle you're leveling, this research you're pouring resources into: you're never incentivised to actually use it.
The solution? Add (substantially) more PvE content OR Modify the current reward structure to make PvP in the late game worth the risk OR provide alternate outlets to use our army - whether it's PvP arenas or whatever.
What isn't the solution? Duke units, new ranks and new races. These are bandaid 'fixes' that add content in the short term but fail to address the underlying issue.
That's my 2c and my biggest concern for what is a game with a fantastic concept and foundation.