I think bonds feature would create an fascinating dynamic quite simply.
A bond would generate $1 per turn;
players may have 'negative' quantity of bonds.
player may not sell bonds when total turn debt exceeds of ?(33%) of tax revenue.

hm; interesting; who would issue the bonds? how would that work exactly?

lets put bitcoins into the game too

bonds are usually sold by the government. so he's basically asking for a "cashing" like function?

Any kind of cash loans would be very interesting.

The bond pool could come from the market tax, bonds can be issued early in the set and have "maturity" dates, more transactions = more tax collected = better payouts, less activity = less tax collected = lower payouts.