Well, that's the thing. In the Multiplayer aspect, it will end up working a lot like, dare I say, Age of Empires. You can set various thing (like fog of war and full map view). You can also turn on and off certain technologies, disasters, the like.
As far as a population limit, I guess if you mean like the standard of 200 units max, that works well enough for me. Actually, it may be a necessary evil. But, just like in AoE, you can set it to 50, 75, 100, 150 and 200. Also, if people have good enough hardware in their computers, they can turn off the population limit alltogether (but again, you're going to need REALLY good hardware for this! Remember, it's 3D and there's going to be a LOT of geometry to render!).
If by "Game Rules" you mean various engine settings than you've got it (including customizable keyboard/mouse operation). The idea is to make a very flexible engine.
If by "Game Rules" you mean making changes to the way the game operates, well... that won't be so customizable in-game. The way to change that is to create a 'MOD' to the game using the provided 'Logic Module' SDK. Simply compile the new Logic Module DLL (or however it works on other systems), put it in a new directory, start the game and load the MOD (from the MOD menu). That's really the only way to truely customize the game's logic, add new units, structures, resources, tech's, etc.
As a side note, the way I envision OP3's engine to work is as a new game but taking the lessons learned from OP2 as well as learning from the success of the Age of Empires and WarCraft series and incorporating all of these various aspects of RTS gaming into one, clean robust package. I hope that makes sense.
