I know they said gates will not be in the game … but what about scenario editor ? You could add gates as a scenario editor only unit . For example RoR has the mirror tower which is basically a tower that fires a red laser been, i assume having gates in the scenario editor for custom scenarios won’t hurt the game.
What about battering rams ? For example AoE2 has fire tower and flamethrower as a scenario editor unit , we could have that for custom scenarios.
Now before you bash the idea , keep in mind that games such as Warcraft 3 live because of the assets that get generated by the modding community with the scenario editor. Giving more tools would help in that way.
Same for land trade units and Trade Workshop (the legendary trade workshop) … that could be a scenario editor unit only … which could offer a long list of possibilities for map creators.
I think saying no to an idea requests so heavily by the community is a waste of opportunity and i thinking having an opened mind for offering assets like those in a safe environment without impacting the core gameplay would be a very smart move - who knows , maybe the next DotA will come from AoE-DE.
On the same idea , please enhance the scenario editor.
That is all 
It would be great to have those as scenario editor only objects.
Essentially, making the game more open to modding might be a good direction.
It would be perfect to use the map editor to generate new playable maps for the community, or new campaigns!
note:
I spoke a few days ago about the workshop of the new AOE DE, if you want to discuss here is the link: https://forums.ageofempires.com/discussion/853/age-of-empires-definitive-edition-workshop#latest
thanks @UnknownMortal , i will look into that
I would like to see gates still in-game personally, but at least if they are added in the scenario editor it would become an added bonus for custom scenarios, agreed 
If they add gates as scenario only, they would need add coding for pathfinding and AI… so it opens the possibility for modders to add them (if there’s modding workshop) to the standard game. No scenario-only gates, no code…
Well , coding wise , i do not believe that would be hard. If i was developing the code i probably make it so that when a gate is open it will be a unit with the asset of an opened gate and it will have a collision size of 0, which , when the path finding protocol comes into play it will find that as a viable path, and when the gate is closes it will increase its collision size to something positive and the path finding algorithm can interpret it as an obstacle. So to be fair, that is easy to.
They can even use the villager as a base for that. For example when you have a villager that is a hunter and has food, that is , let’s call it a villager_hunter unit , but when you right click it to gold, it changes from villager_hunter to villager_goldDigger (hence why no female villagers - pun intended - sorry). This is an example of how an entity changes typeRef on the fly while in game. Same principle can be applied to gates.
For a moder to achieve this , you need to do a lot of mini-hacking , but for a source coder that has access to the base code , it should be a fairly easy addition.
And if you really want to nit-pick -> AoE-DE is using Genie Engine as a base, the exact same one as AoE2. So if it can handle gates there i am more than positive that copy-pasting some code from AoE2 to AoE-DE for gates is kinda easy.
You do not need to add AI all that much as it will be a scenario only unit , so AI does not actually build gates or use them all that often. You can even make triggers to toggle gate status if you really want to in the scenario editor. I mean solutions there are, as long as the developers offer us the tools to play with it.