Looking at some of the strategy games that have been successful - Civ2, TOAW, Combat Mission - they have had good community input through new scenarios. And looking at an era I have not hitherto been interested in - Modern Air Naval Operations - seems to have a lot of following on the Matrix site with references to many new scenarios being made.
AGEOD has a game system that covers Ancient to early 20th century.
You have great maps for most of Europe to the Middle East; a database of units and values, and a game system that works.
Why not make user made scenarios easy?