I just wanted to update this thread, since there is often a feeling that there is very limited co-op content available. A few people in the community is doing a lot of heavy lifting converting old campaigns or community campaigns. On Reddit there is a list over all content that have been converted: https://www.reddit.com/r/aoe2/comments/1giii85/aoe_2_coop_content_grand_overview/
A modder like @JadenVance is still very active and @Krambas recently update his co-op version of Joan of Arc.
A good way to support the modders is to do like @nirvana11 and give feedback on bugs in the conversions: Mihira Bhoja (2P Co-Op) - #10 by JadenVance