I would like to do something like
map <C-k> :e#<CR>
in my .vimrc.
However, I recently found that <C-k> is used for digraphs.
How do I get a list of unmapped control keys in vim?
Vim has many commands, so it can be a challenge to find a (memorable and short) key sequence for mappings.
My approach is to use the recommended <Leader> prefix for things I do not use frequently, but for essential stuff a mapping with Ctrl is useful, indeed.
Learn how the commands are represented in the help (e.g. CTRL-O in normal mode, i_CTRL-X_CTRL-N for insert mode), think of a good mapping candidate, then try to look it up via :help CTRL-k) If there are no matches, you can make sure that the mapping is free via :nmap <C-k>; if there is a match (the nice thing is that this also covers plugins that supply documentation), you can read the description, have at least discovered a new Vim command, and can then decide whether you need it (then retry with a different mapping candidate), or whether you override the command.
(Note that you can also :noremap built-in commands to other keys, but be aware that this makes you increasingly helpless in vanilla Vim installations or other application's vi-emulation modes.)