I would like to quickly access my message history like you can on a Terminal in case you need to retype the same or similar thing repeatedly (not for spamming of course 
).
I think SHIFT+UP and SHIFT+DOWN would be quite obvious controls for that (as UP alone is in use already and I feel CTRL/ALT/CMD are better used for more global functions, like channel manipulation, etc.)
A suggestions to avoid collisions with browser or OS-wide shortcuts: maybe use VIM style direction keys ( e.g. h,j,k,l,gg,G ) with a modifier for switching channels, mark sth. read, open a channel or direct chat, etc.
This could be extended to :command style syntax within the text field of any chat.