For an expansion they should consider integrating a system where you can invest your gold into civ-wide benefits. For example, being able to invest (using fake numbers here) 10g per turn to obtain a constant military bonus (+1 str to units, something silly like that)... as long as you continue to invest the X gold per turn, you gain continue to gain the effect. You could have multiple trees/branches, similar to the policy system, but rather make the trees more linear. Have trees emphasize military, growth, etc... as you progress down a tree you reap greater benefits at the cost of greater gold per turn. For example, Military Bonus #1 (1 str to units) would be 10gpt, but bonus #2 might be a total of 20-30gpt.
With multiple trees, and the earlier bonii being cheaper, this would allow choice in how dedicated you want to be to a certain kind of investment. If you're serious about investing in superior military, you just may be prepared to spend tons of gold per turn to reap those specific bonii.. However, if you'd rather spread that 50gpt around, you could get lower levels of various different investment opportunities.
I think something like this, if refined and actually worked on as more of 3 minute thought, would strengthen the value of the dollar... I mean gold. (that was a joke)
Oh, and it also gives the choice of whether you want to hoard gold or spend it for civ-wide bonii. If you begin to lose gold, and reach zero, you'd be forced to abandon a particular investment, and thus lose the bonus, while simultaneously bringing you back into +g -- which would actually create the dynamic of hoarding for a period so that one might invest for a period later... for example, if (still using random fake numbers) it's 100gpt to invest fully in the military tree and gain all of it's cool goodies, but you only make 50g... you might hoard gold for 20 turns to get a surplus 1000g, then declare war, being able to fully invest in military bonii (100gpt) for 10 turns.
In fact it could even be designed around the concept of "temporary buffs" to begin with... since such concepts are already in place with research pacts and golden ages (which aren't gold related... but ARE a temporary buff). That could give research pervue of civ advancement, culture dominion of persistant bonii that affect your entire game, and gold the domain of investing in short term boosts (which requires diverting gold from what you'd otherwise regularly use it for).
could be interesting.