The major side-effect of tweaking the Massive Damage save is that high-level spellcasters become a whole lot more powerful.
For example, by 11th level, the wizard can throw a Maximised Fireball for 60 points of damage. If the damage isn't bad enough, you have to take a Fort save or die anyway. Granted, you get a Reflex save, but if you fail that, you are on for a MD save.
Now, under Crothian's ruling, that DC becomes 25 (!) Compare and contrast with the higher level Finger of Death (which has a DC probably in the low 20s, affects one creature and does much less damage on a failed save).
Under uv23's ruling, this save becomes nigh impossible to make. 60 points of damage? So, with a Con of 20, you still have to make a DC 40 save. The 2xCon works for a bit, but when the Maximised Cone of Cold hits you for 90 points, you will be facing a DC 50 save (assuming 20 Con doubled).
Hollywood: What of spells? The 'base damage' theory is all very well, but if applied to spells, what would one use. For the Maximised Fireball, would it be 15+d6, 15+6, 15+10d6 (!) or 15+60 (!!).
Personally, I think that the MDT works fine. If any of the tweaks here is passable, I would probably run with IceBear's.