The lack of answers to this questions is rather concerning. The creators of our community either do not care or do not recognize the value of their contribution. Be assured any work you do on this game has creative value and you own a part of it (even if you’ve taken the art and music from elsewhere ).
Either way, I would encourage you to add license text to your work. A license is a way to communicate to us, the users, how you want us to treat you creations. Do you mind if we take your work and expand it, do you want us to acknowledge you as the author, or can we do whatever and just leave you alone?
There is no need to be afraid of the lawyerly aspects of it, there are plenty of simple, open source, community focused licenses out there, that you can copy and paste as a text file to add to your mods. If all you want is to share your work, great choices would be the Creative Commons license (for artwork and content), BSD, MIT or GNU LGPL license (for software such as RMS).
Microsoft does need “copyright agreement” to distribute the mods. That most likely is covered in the EULA. However, Microsoft cannot demand full copyright control. Even if they do, it would be invalid.
A lot of creations are completely independent. An example are map scripts. Map scripts are programs. They require an engine to be interpreted - a version of Aoe2 is likely the only option for that. But that doesn’t matter, the scripts are the IP of the creators, just like web pages are not owned by Google or Mozzila. As such the authors have full control over them. (This is more limited than they would probably like. They are owners of the specific script, not the idea of the map. Anyone can write their own script that would generate very similar layouts, no copyright issue here.).
Some creations are also derivative. For these works Microsoft holds the copyright for a part of them (graphics, sound, etc.). These are probably covered under Microsoft’s Game Content Usage Rules. But these rules only cover the media owned by Microsoft, and how you can use them. It does also add limitations to a license you can use for your creation. But the parts you added are still yours - you have copyright over them.
One simple example, that Microsoft likely doesn’t care about, is if the author of a mod permits derivative works and if he wants contribution. Microsoft probably doesn’t care about that one bit. Anyone modding likely won’t think twice of looking at a mods files and modifying them. But it should be the authors choice, if he allows this or not.
TBH my interest in modding Age went down to zero. At least on HD, you could easily try out if your mod worked or not. On DE, it’s a mess.
Without Independent Architecture support, officially or unofficially, I don’t see myself modding the game.
A shame that they simply couldn’t/(didn’t want (?)) to simply update HD with all the great stuff DE gave us.
I wish Microsoft allowed modders to be able to share a portion or revenue generated by their own work with Microsoft–like an Apple App Store for mods. If I spend 300 hours making a mod and want to sell it for $2, I get $1.33 and Microsoft gets $0.67. They are myriad benefits to both sides for doing something like that.