Idea: Semi-apocalyptic, management/strategy sim.
Premise: World is ravaged by a bio-engineered disease that kills 90% of men (particularly older men, with young boys being either immune/able to easily fight it off) and 20% of women (particularly older women). Everyone left alive is immune so the virus is a non-issue. This resulted in a war that broke out along the border of your country (invading country tried to use it but lost control of the virus. Ended up backfiring on them and now there is a stalemate). Boy (MC) is separated from parents at a very early age as the war breaks out and they are enlisted and society breaks down. He lived with his big sister for 5 years to get by in a camp. During prologue, camp is invaded by opposing country's forces (consisting almost entirely of women at this point). Taken prisoner only to find out that the leader of the opposing country's military is actually his mother, though it is hard to tell at first. (could add a plot line that slowly reveals this over time, or just open with it). The mother joined the other side when she found out that her own country was the one that released the virus originally and it killed her husband. She rose up the ranks quickly due to her determination and strength.
Now, you're tasked with invading and controlling various cities on a map. Characters can be deployed as "heroes" to help certain operations (like defending cities or invading cities, carrying certain attributes that help your forces) As you gain victories, you open certain scenes with the characters. Losses will also open other scenes (possible NTR).
Mechanics: Many ways to do this. Generally would need a UI that has your country's resources, income/operations. Since males are rare, there is a certain "pent up" demand for various services that the remaining women desire. Seeing as how MC is one of the last males around, prostitution becomes an option to accelerate cash. Sister could be good at espionage and spotting troop movements while mother could give invading forces a bonus. (Could meet other characters when certain cities are captured, with other modifying abilities).
The very simple way to do this is to use cash to: Hire troops, upgrade equipment/morale (stats), improve economy/production. Additional Cash can be gained through prostitution (MC), invasion (Mom), or espionage (Sis) but all come with risks.
Endings could be initiated by:
1. conquering all cities
2. losing all your cities
3. other event (maybe there is a time clock where the opposing side develops another, better manufactured bioweapon that really eliminates everyone on your side. Two versions are one where everyone dies or if MC or other characters surrender/are held as slaves)