If you're on mac, Karabiner is the best tool for remapping. I use it to remap normal keys to hyper keys. So pressing on `a` key together with `f` will do control + f and things like this.
I tried remapping a normal letter key's (a) long press to a command+a but it seems to actually trigger the letter "a" before triggering the modifier.
* Mapped both caps lock and return key when they are clicked with another key to act as Ctrl. When they are clicked alone without combination then they work as ususal.