bright plover
It seems, many players already had to face with such problem - teaming.
And it can be in many forms. Mostly these teamers are internet friends and coordinate their actions through voice chat.
Or these teamers are situational allies, because you've became too big.
We will talk about both cases, because they are mostly similar in details.
Embargo
In any stages of game (beginning, middle or endgame) trade is the key to win. Not always, but in most situations. When you can trade you can save up enough money for such things as buildings or nuke weapon.
Trade plays an important part in a gameplay.
But what if you are under embargo and can't trade, like, with everyone? In endgame it becomes quite a serious problem. And it makes things more difficult, because after you've got embargo from every player, you can't save up money effectively. No gold - no win.
Solution
• Railroad
It's not a full solution, but a part of it.
Such structures shouldn't be like buildings, but as lines between cities. Railroad increase income from workers and slightly increase population cap.
You can connect all your cities, plus, connect all railroad system with ports, what will increase income from trade a little.
• Factories, manufactures, oil rigs
And etc - all this is industry and main part of economy. It will increase total income much more and will compete with the traditional method (trade).
Factories and manufactures are one building on different levels (devs are going to add upgrades for buildings, so it will make sense). This is active income and part of domestic economy. Also it can make some buffs for your army, but not much to do not make it unbalanced.
Oil rigs are passive income and give some buffs for factories. Plus, they give buffs for trade.