I'd rather the base layout and shifted layouts were standard with international stuff behind an unmapped modifier. Let me explain why.
This is the finnish keyboard layout:
Notice first the Alt Gr in the place of right alt. That is a modifier key that shifts to the right hand side blue characters. Also notice all the dead keys on the right (red).
Now, consider an application saying "next weapon is mapped to ]". Which of the following works?
1) ä, 2) AltGr-9, 3) both, 4) neither
The answer is: depends entirely on the application. I've seen all of the above.
Now consider a common "console key", tilde (~). Which of the following works?
1) §, 2) AltGr-¨, 3) both, 4) neither
The answer is: usually § or neither.
Now see all the stuff in messed up places and behind AltGr. I've seen applications that try to do their own keyboard handling that are not able to produce any of the characters behind AltGr. It wouldn't usually matter if it was just ÅÖÄ behind it, but @, |, $, ;, :, [, ]?
Many times I've had to just live without some keybindings or shortcut keys.
This is why, for consistency, I propose keeping the base and shifted layout the same for everyone, with any user-dependant characters behind a user-mappable modifier key.
EDIT: One could say "Why not only map the few characters? You don't need to remap all the keys!", but then the question becomes: "Where do the overridden characters go? Behind another shift?"