A stock market would be interesting but since most people usually just keep their money in items it would give the few people who keep tons of money way too much buying power. I think it would work better where you put in a few items (e.g uranium or smt) and you have to interact with it every other day in order to keep it growing as fast as possible.

For instance:
an "overheating" system where after a day it reaches 50% heated and you have to cool it down, with another 2 days where it slowly reaches 100% with diminishing returns depending on how overheated it is.

There could also be different minigames at different levels of overheating, for instance at 20% and 40% you have an opportunity to get extra rewards but have to catch it between 20-30% and 40-50% heating respectively.

You would have to end your current session in order to cash out, with an exchange including high-mid tier weapons and suits and a stable cash exchange rate - but this would have to be relatively low, tanking the price of mid tier weapons and suits but not overinflating the economy.