Yes, players will be able to use heroes or units of other factions. 

• You can use all your units from a different faction, but you might not be able to upgrade units of a different faction, except county unlocked units if you are on a county of the faction that unlocks this unit. 
• Heroes can always be leveled up if they have enough XP. 
• The gathering units (Miner, Huntress and Mounted Lumberjack) are factionless and can be upgraded if the player has the given research.