Most of gold features provides uneven superiority over others.
Can't say i'm talking on behalf of everyone, but for me such features spoil the fun of the game.
Here are some optional feature i was thinking about that can encourage player to buy gold/spend gold without breaking equilibrium:
1. Buying packaged features such as Investment vouchers, Instant dispatches, Licenses etc. (most things that you only get from lottery)
When you buy a package you will receive and extra item of it, or it can reflect some kind of discount.
2. Corporation/city fund raising of gold which will unlock bonuses or benefits for related players. (such as overtime in facility/commodity chain benefits/goods value bonus)
3. (1) Facility investments will be gradually reduced if facility Occupancy Rate = 0 -> (2) Gold can prevent the individual's penalty.
4. Disposed city good can be resurrected for certain amount of time.
5. Gold can be used to reconcile lack of money for investments for a commission.
Example 1: I need 1M for investment, i only have 800K, for the missing 200K i can pay in gold, 20% of investment equals to 20 gold +commission in gold. (amounts specified are arbitrary)
Example 2: I want to place an offer for a license, 300K, i only have 250K. 250/300 - 17% of investment is missing, this is equal to 17 gold + commission.
The idea behind this is not to replace the need of money and the hard work of earning it.
6. Having and extra level for Rail station buildings that can only upgraded with gold. (only available after finishing upgrading the building to max level)
7. Scheduled construction: Let's say i'm away for the next 6 hours, i know that while i'm offline i will have the required amount of money to upgrade a train station building. for a fee, i can schedule an upgrade once the required amount has been reached.