I like to keep things short.
It would be of great use in informing your players to create a sticky topic where the OP is edited to show what you are currently planning on implementing (concerning game aditions).
Allow users to customize their new game. You already have a difficulty model, all it needs is a “custom” subtype that allows the player to set certain attributes. Starting funds and calendar speed are the most important.
I’ve read it stated that you wish to enable the modding community. It would be in your best interest to allow players to keep the original game intact. We can edit certain settings now through the config files, but to do so, we have to backup the original settings. a structure of (mods\modname\<custom settings>), allows the player to pick which modified settings to use when starting a new game. Since the original game’s settings are not changed this way, players can save games for different mods and the original game; all of which will be playable without moving things around in a folder. This is also a first step towards a framework that lets modders alter and add game mechanics.