How remap the CapsLock key to work as if it was a Return key using setxkbmap

(This answer is about xmodmap only. I'm sure it's possible to do this with XKB, I just don't know how.)

Modifiers and keysyms are assigned independently. But you get strange effects if you don't set them consistently. I think all you're missing is the add command to assign a modifier to Meta_L, though you may also need to clear and reassign the modifier keys. You may replace Mod1 and Mod2 by Mod3, Mod4 and Mod5: they are interchangeable, just make sure you don't use one for two different purposes.

clear Mod1
clear Mod2
remove Lock = Caps_Lock
keysym Caps_Lock = Alt_L
keysym Alt_L = Meta_L
add Mod1 = Alt_L Alt_R
add Mod2 = Meta_L Meta_R

I have the same issue. From the doc, one could guess that the correct syntax would be something like that :

$ setxkbmap -I$HOME/.config/xkb -option mine:accent

Note : ~ doesn't work with the -I option

with

~/.config/xkb/symbols/azerty_altgr

partial default
xkb_symbols "accent" {
    include "level3(ralt_switch)"
    key <AE02>  { [  NoSymbol, NoSymbol, eacute,      NoSymbol ] };
    key <AE07>  { [  NoSymbol, NoSymbol, egrave,      NoSymbol ] };
    key <AE09>  { [  NoSymbol, NoSymbol, asciicircum, NoSymbol ] };
    key <AE10>  { [  NoSymbol, NoSymbol, agrave,      NoSymbol ] };
};

or whatever symbole file you have and

~/.config/xkb/rules/evdev

! option        =   symbols
  mine:accent   =   +azerty_altgr(accent)

Unfortunately, it doesn't work.

The only solution I found is the following : Use the same files described above. Set all native option using setxkbmap -option caps:swapescape,whatever. Then :

$ setxkbmap -print > tempfile.txt

and edit tempfile.txt in order to add your custom option in the xkb_symbols line :

xkb_keymap {
    xkb_keycodes  { include "evdev+aliases(qwerty)" };
    xkb_types     { include "complete"  };
    xkb_compat    { include "complete"  };
    xkb_symbols   { include "pc+us+fr:2+inet(evdev)+capslock(swapescape)+azerty_altgr(accent)"  };
    xkb_geometry  { include "pc(pc104)" };
};

and finaly :

xkbcomp -I$HOME/.config/xkb tempfile.txt $DISPLAY

It can be generalised for any number of options. This solution is deprecated according to Archlinux wiki.