The real problem with trade

Why do you think being able to transport more gold is going to help?  The very desire for such is only indication of how much gold's value has plummeted, and it needs value restored, not perpetually allowed to spiral further downward by moving ever greater quantities.

Gold is not small.  Gold the same size in units as every other resource, and being defined arbitrarily as a number like every other resource, does not need to complicate transportation systems.  It is however worthless, which makes us think of gold units as being some tiny quantity.  How small does a unit of gold have to be that it be deems worth less than a unit of wood?  Gold is normally worth far more whether compared by mass, volume, shininess, or the gleam in a gold digger's eye.

But how can gold be valuable at any relative size when we have so much of it?  The other basic resources are spent far more, yet gold is the resource we can gather the most by at least an order of magnitude.  It won't matter how much we can move or spend it unless the desirability of spending it actually exceeds our ability to harvest.  (Remember that by natural, intuitive cost-benefit analysis, as we gain more of the things for which we can spend gold, the price we are willing to pay for more decreases--and more so when currency supply dwindles.  Equilibrium is not reached until our strong appetites exceed our ability to sate them.  Income has to be matched by expense before we find the prices too steep.)  Given how much taxation can increase income, that would be one heck of a continuing long term expense.  Even the desire to hoard a resource dissipates when we realize it doesn't hold its value anyway.

No one is hording gold though--why would they?  I have a ton, and I'd gladly spend it on everything under the sun because I have so much and it is useful only as a means to an end.  But of course no one wants it because it's as worthless to them as it is to me.  At the quantities I'd have to spend to gain anything of worth, I might as well just keep it for upkeep buffer on a rainy day, like if I'm besieged and suffer a major population drop.  But that's not the same as hoarding.

Consider mercenaries as a spending option.  At ten times the gold upkeep, I wouldn't use them regularly.  At 20 times the gold upkeep, I wouldn't use them ever.  But after weeks of constant production, I could still easily double my army and handle the upkeep without a negative balance.  To me--one of the largest players (top population) in the largest alliance with the highest potential income--hiring mercenaries is the least necessary.  But you smaller players are welcome to halt your economic growth to fund a war against me while I continue to grow and match your armies with drastically lower taxation.  And if I do get in a pinch, maybe I'll hire 10 times as many mercenaries as you for a short-term operation.  I'll even take fewer losses having my force concentrated into a single attacking army.  If you want to lessen the imbalance between players, increased spending options will do the exact opposite.  Scaled income will do neither.

A self-balancing system like I propose is pretty well guaranteed to work (by which I mean set a slow-moving and very reliable value for gold, based whatever is defined as the per-unit population circulation) and does not alter the existing range of variance in player incomes, regardless of existing stockpiles.  A system based on increased opportunities to spend gold into circulation sinks might work on its own as well.  Possible side effects include breaking other balanced constraints, particularly turning military equipment into unlimited* resources for everyone developed enough to stop building a city and crank up taxation...that would kill trade like nothing else possibly could.

*by unlimited I mean able to be acquired faster than it can be spent on unit queues and without aid from other players or supporting towns, making every developed city fully self-sustaining without specialization.

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I should also note: scaled income as the balancing economic factor wouldn't have nearly so large an impact on current incomes as you seem to think, col.  In fact it would even increase incomes if the balance point isn't reached yet, and it quite possibly isn't.

Let me paint the picture a little differently.  Suppose everyone has upkeep and normalized production expenses equal to their income.  This is an unrealistic state of perfect balance, but my point is that at this state, with a current world population of 2,314,975, everyone can have their cut of the world's global supply of 231,497,500 gold as an operational buffer.  Assuming perfect distribution, that means a 6,000 population player has 600,000 gold to smooth out market fluctuations.  That's probably actually too much and perhaps the gold/population factor should be more like 20 instead of 100 (giving a 6k pop player 120k gold).

The more I think about it, the more convinced I become that you cannot have a proper economy without a properly limited resource.  Until you know with reasonable accuracy and consistency how much gold is in the world, you don't know what a given portion of that gold is worth.  Anything else is just a perpetual arms race, and people don't trade during an arms race--unknowns aren't trade-able, and producing your own is always fastest.

Gold may be small but it weighs a lot......i value gold in favor of 2 to 1....i offer 6K of gold for 3K of resources...works for me as all my trades go...