I started making a few issues in the GitHub repo, so if there's nothing else set up yet it could serve as a preliminary to-do list, and eventually if things pick up it could be used to track outstanding elements and bugs during development. So far I just plotted down what I could think of but it would help organise the different things that need to be done, and each issue could serve as a tracker of whether or not it is possible in the game or not yet, and which mods could be used to support it. For example, as you mentioned an inventory system, this issue could be tracking the discussion on possible ways to use it in the map, and possibly link to upstream tasks it is waiting on in the 0 A.D. source tree.
The repo and organisation on GitHub is just something I threw together quickly so I'm just putting it out there as tentative place to collect ideas and drafts, but let me know if you want access. I think anyone can comment and add issues, without any special privileges.