I have a big Colonial Tycoon suggestion for an endgame, but it might not be very easy to implement.
I have played through until the end and unlocked all of the available technology, and now there is nothing really left for me to do in the game. It's been fun while it lasted, but now there isn't anything to keep me coming back to the game. Some other "idle" style games use some sort of "restart" mechanic to add in some complexity, but I think this game could be fairly unique if there was some indicator of market forces at work once you get into the endgame to add some challenge and unpredictability to the game. It would be something that could keep me coming back for more, even after all the available technology was researched, and all businesses were running with Max Level and Max Managers.
Here is what I am thinking - After a player has researched all currently available technology, the game enters a new mode where instead of a fixed amount of revenue, the revenue per business is variable based on market demand for that particular resource. (Basically, a positive or negative multiplier, which is randomly generated, is applied to the revenues of a particular industry). The Market Demand can go up (and the player would see "News" articles like, "Housing is estimated to be scarce in the coming months, prices are likely to increase" or "A new industry starting out is going to drive up the prices of Ore") or can go down, ("New retail competition slims down profits for sales at local Megamart" or "Tourist dies in an accident! Tourism revenue expected to decline rapidly"). These messages can be randomly selected based on the positive or negative effect on particular businesses. The "News" articles are displayed in a popup on the screen so the player knows what to expect as the prices change and helps explain what to look for. (Side idea, these could be archived and can be seen using a new "News" window to review past articles).
These market changes might mean the player would be wise to sell off businesses that suddenly start costing so much more to run that they go negative, or buy new ones where the profits are sky high. The changes could be sudden (a quick spike or fast selloff), or gradual (slow increase or slow decline). Changes to market forces could happen at a fairly random pace (likely with bounds for how little or how much time must pass between changes). This level of unpredictability would mean that a player would constantly have to readjust their business holdings to ensure they remain profitable.
I hope you enjoy my idea. If there is anything I can do to help with making this a reality I'd love to be of assistance.