Somewhere, the option to map a combination of key presses to some other output, as can be done in Microsoft Word. The biggest use I would have for this would be remapping two hypens to an em-dash, since for SOME reason keyboards don’t have an em-dash key built in.
This could be part of the spell checker, though I personally would like it as a separate option/plugin, for use in code, or just for filling out complicated key formations.