Look at the U.S. before the housing bubble burst for example, everyone was buying homes, and they were doing so with rates as high as 18% interest. This rate was due to inflation, the more people buying on borrowed money, the higher inflation is, and thus loan rates will be high. In the NES if everyone borrows money on a given turn, the rate of interest charged should be proportionally higher. So, 10/10 companies all take out loans, then interest would hit a max cap of say 22%. Should the businesses decide not to take out loans, the bank (me in this case) would lower rates to encourage people to take out a loan down to 2%. This low rate will encourage additional lending so that the bank will be profitable.
That's the nuts and bolts anyways, in practice the NES isn't going to have any outright war, so the only way to fight is going to be through hostile takeovers, stock prices, and overall growth, which can be influenced easily by stories. As a moderator, one would need to update stock prices based on story... actually I'm complicating it a bit too much now.
Story leads to business success, which leads to buying out smaller companies (competition) but better stories from smaller companies can cause greater success for a smaller company, leading to even competition.
I could maintain an excel spreadsheet that would track company stock prices and outstanding loans, as well as stocks owned by each player fairly easily though. It just sounds complicated.