It would be possible to create new clans, like nomads. Until they distinguish themselves in some tournament, they would wander the lands... and then settle somewhere. It would also be possible to divide the territory into sectors and create a map for each sector in the editor, and then play on it. Neutral territories could be conquered through official tournaments... or like this: you openly state that you want to occupy a neutral territory, and if someone wants to interfere, they interfere. It would also be necessary to set a limit on the capture of neutral lands: say, 1 square per month.

Added 8 minutes later
As for new lands: they will eventually run out anyway... this is the world map of Heroes, so you could put a portal in the middle that leads to a new map...