It would be really cool to be able to link two or more cards together visually on the Trello board,
I know it's possible to link one card to another by adding a URL, but this doesn't affect the state of one when the other one is updated.
What I really mean by this if you create a task for yourself to work on that you need to track, but also you are for example using the Trello email add on where you can create a card from an email you've received which related to the task you created, you want to see both visually on the board they both relate to the same thing and you're keeping track of them with similar progress status.
Which means you can actually drag the two cards together into the Done list for example to update their status and you can see them both move together.
Who's with me?