I've been experimenting with a TODO for every branch and then merging the TODO into the main TODO when you merge to master. For example, you have a main TODO that has the general things you want to do. When you open up a new branch, you create a new TODO file named after the branch in the TODO directory. You move the TODO items that you are going to do into the new file (and add extra details). You work on that TODO and when you are finished with the branch, you move any not-DONE items into the main TODO file. This will cause occasional merge conflicts, but since you should probably inspect the priority of close items, it's not really a problem.
I've mostly done it just for myself, but have occasionally tried it with 1 or 2 other people. It seems to work reasonably well. The only real downside that I've found is that if you want to take the TODO items out of the master branch when you are working on them, it requires another commit to master, which is unfortunate. However, in doing that, your master branch has a TODO file for every branch that it in active development, so it's not really that bad...
I've not really lured anybody away from Trello, though. People like their specialised GUI ;-) Org-mode/MD just doesn't do it for a lot of people.