It takes a fair bit of work to set up: the fixkey documentation has guides for each popular terminal type, including tmux and screen. If I understand the source correctly, it seems to map new keys to
<F37>, and then
<S-F37>, giving 50 possible keys.
Once it's set up,
:nnoremap can be used to map previously inaccessible metakeys, just like gVim.
nextfile (GitHub: bighostkim / nextfile.vim, License: MIT) by Allen Kim is a script for quickly switching between sets of files. If you're fed up of navigating between a file and its associated tests then nextfile may be what you're looking for.
It uses a function called
AddNextFile that takes in regular expressions to determine how to switch between files. The example in the readme is for a Node project, but it could be easily adapted if you
s/js/py or whatever language you use. There's even an elaborate example for the rather baroque way Rails works.