I adapted my proposal, looking at Saber's proposal and ED's remarks about it. This is the resulting compromise:
http://www.keyboard-layout-editor.com/#/layouts/96ce05fe6ad1693cb29247f4d18b7974
Is there any reason for not having AltGr and shift in the same side?
I would greatly appreciate having them one above the other (shift above AltGr) to be able to:
1. Map additional keys to AltGr+shift+key
2. Use always the same thumb for the modifier.
Rationale:
The first one is to enable the user to use xmodmap to reconfigure the keyboard for the non-supported languages -- in my case, for Spanish, I would like to map 'ñ' to AltGr+N (and 'Ñ' to AltGr+shift+n) and also move some symbols out of a key to have a dedicated dead key "acute accent" (´). The moved symbols could be relocated to AltGr+Shift+some-key.
The second one is to (IMHO) fix a shortcoming on the Pandora's keyboard
. The Pandora has Fn and shift in opposite sides, and it is annoying when you aren't sure of the modifier to use to enter a specific symbol --as in: "was left thumb + 7 or right thumb + 7?" you try one, fail (Murphy?), and have to reposition both hands to try the other combination.