Regarding the burning buildings feature in AOE 4, take that with extreme caution.
A unit with the hability to set something on fire, can turn the game in a firefighter game.
Something that i think could be used on AOEII, is buildings with 40% or less HP, starts burning and losing X amount of HP per second, increasing with the lower the building’s HP. That would be a nice feature, to remove the ever lasting burning buildings.
Reaching 10% or less, could start burning the near buildings if they are next to each other. This would be awesome to wallls, maybe this way, players would stop using houses instead of walls.
But units to set buildings on fire could be dangerous to the gameplay, unless it uses the system mentioned, losing a minimum HP if has full HP.