Oh, and of courser not every stone quarry should offer an entrance to a cave system. It must be discovered. This might be an option to mine more stone over a longer period. In a typical game, stone appears to be a resource that is used in larger quantities mostly during build up.

The difficulity is of course the actual game mechanic: a building to walk on, not like a wall, not like a field. For sure not easy to implement, hence this might remain a dream. Ah - one more I remember from Settlers 5 that was fun: - tunnels/caves - you could discover and enter them, and then surface again on another place on the map (teleporting is a feature we have already iirc). Of course this works for both sides. To add more fun, the entry would be blocked by stone (a stone quarry). Once the stone is removed you have a secret passage to another part of the map. Watchout - it can also be used by your adversary if he discovers it!

This would be pretty epic, I always wished we could build the bridges from the scenario editor is Age II. Roads would be an interesting feature, most useful I think for shifting forces around your territory I would think, though aggressively building a road into your opponents land would be terribly funny as well  

Sure, this is the best source I have been able to find so far, if anyone has a better source please post it. Putting it as a mercenary unit could definitely be one way of doing things https://warhistory.org/@msw/article/war-elephants-in-the-roman-army

If I may dream: Walls that protect against projectiles if behind a wall, except for "artillery" Abitlity to build a bridge (Settlers 5 had that) - this could preconfigured sites on the map where the path crossing a river or so is initially blocked (like a closed locked gate) but "somehow" indicating a bridge could be built here. Building a bridge basically means creating a building animation and when finished opening the path ("unlocking the gate") thus allowing crossing the bridge - similar to opening a gate in a wall or palisade. The mechanics must however be inverted: default closed for everyone. Only if the bridge exists the path is open - but for everyone. Interesting gameplay could be that you could destroy bridges as well thus closing the path again. Ability to build roads that have a speed aura for everyone following them. Adding water as a kind of resource - i.e. building a field in the desert does not make much sense unless you are close to an oasis (open water body) or build a well or fountain to get water from underground (romans of course build aquaeducts ). Crop yield could be low if no water with a limited lifetime of the field, and high if water is provided by a nearby source. For existing maps, the well house is the only possible source of water. For new maps, an area around water bodies (oasis, rivers, lakes, ponds...) could be defined that allows farming. On desert maps this could force players to fight for water access. 

You should show sources, it's not that I don't believe you, but it seems more like a mercenary unit to me.

The Imperial Era does not feature here, we talking about the time frame from the Second Punic War up to the first century BC. I believe there are enough wars in that time frame to make the justification.