Soldier's diary on inventory screen.
Right now soldier's diary can be accessed only from Bases --> Soldiers --> Soldier screen, which makes diary almost unusable in decision making process. The only time you really need soldier's diary is when you are in an inventory screen equipping a soldier with appropriate stuff. But you can't access soldier's diary there. I very strongly wish you could.
The most logical place for a new button (in my opinion) would be inside "additional actions" menu that is available if you enable "additional controls" in game options. That will leave base UI unchanged if it is a bad idea to put there another button. It would be nice to have a key-shortcut for soldier's diary as well.
