"But we’re also experimenting with new game modes in Age of Empires 2, so there might be new game modes in Age of Empires 2 as well."

In a new Interview on https://forums.ageofempires.com/t/new-german-interview/63982/2 about AOE4 there was an interisting part about AoE2!

In Age of Empires 4, you can look forward to all the modes you know and love from the earlier parts. That does not mean that we do not try new things. But we’re also experimenting with new game modes in Age of Empires 2, so there might be new game modes in Age of Empires 2 as well. And if people love that and the response is there, then we’ll probably bring that into Age of Empires 4 as well.

More can be read here: https://old.reddit.com/r/aoe4/comments/e4gxs6/new_german_interview/

So, if they add more gamemodes to AoE2, what do you wish for?

I think CBA is a no-brainer since that is the most played non-classic gamemode. But there are other popular scenarios such as zombies awakening.