Contracts
We all know you can buy industries (which are way too cheap in my opinion). Another option I like the idea of industry contracts. You buy the right to ship x number of goods from a supplier. Doesn't matter who supplies the raw materials. While the contract is good the # of goods at the location will rise until ONLY YOU ship them. No poaching. As you upped the difficulty the contract would be void if failed to ship goods in a timely fashion.
Market Swings
Now this is one that I'd love to see change as the difficulty goes up. I think there should be more of a boom or bust cycle. For example: Lets say you rely on passengers for profit and your opponent is relying on food. If a food bust event happens then (as the difficulty is raised) the financial impact can be massive (major losses when shipping the good). While the game lowers passengers as time goes on, it should also lower passenger destination (long distance revenue) too. Could you imagine a senerio where you have to ship x goods/passengers to y even though it's very unprofitable? You need other operations to pay for it (then hope no major nasty event happens). Good events should have the opposite effect.
City Offers
While 100K is great early game, as it moves on -- oh well. How about offers like 2 shares of x's stock (if available) or an exclusive contract for x years with a bonus? How about mid game a city offers a million for 20 goods?
I ramble....
Still fun to play...