Open linked card as in-place card instead of opening whole new window. To open in other window add link "Show this card in usermap".

Example:
I have two user maps "First" and "Second". Now I want to link card named "Previously done feature X" from Second to a card in First. I want to be able to easily view content from card stored in Second but not open separate window unless I want to.

More UI ideas:
* Card could be opened as a stack of related cards
* Card opens as second card on top of current one, in large displays you could show many of them.
* cards could be opened like a photo album