I would suggest implementing a system where, for certain factions, players can choose between different upgrade paths for certain buildings. For example, you could choose to build either an assassin building or a troglodyte building. Once one building is constructed, the other becomes unavailable (similar to Heroes 4). This would resolve the ongoing debate. Many people suggest adding new units, but there's nothing to remove. However...
I would suggest creating a swamp faction and adding hydras to it. I have an idea for 7 units for this faction, but this topic isn't suitable for these discussions.