D&D 5e is set up so most characters only have a strong need for one ability, and then there's a few that you don't want to ignore but don't need to really focus on, and stuff you can dump. I wonder what the game would be like if MAD was really for everyone. My ridiculous model would be:

• Strength: used for damage rolls with all weapons, as well as spells that use attack rolls.
• Dexterity: used for attack rolls with weapons and spells.
• Constitution: no change.
• Intelligence: used for save DCs and checks for Conjurations, Evocations, and Transmutations.
• Wisdom: used for save DCs and checks for Abjurations, Divinations, and Necromancy.
• Charisma: used for save DCs and checks for Enchantments and Illusions.

That obviously changes a lot of builds, and would generally make PCs somewhat less powerful (fighter types would need both Str and Dex, casters could make do with only one mental stat if they severely limited the types of spells they used, otherwise need multiple), but I'm wondering if it would make a good variant. Thoughts?

I think it's an overcomplication, results in less interesting builds, makes levelling less fun, will be the end of feats because all your ASIs will be going into stats instead, and will confuse most people because most of your save-stat matchups don't make even the tiniest lick of sense, would be a massive undertaking to employ requiring a complete rewriting of every single balancing mechanic in the whole game to a point where you may as well just go make your own D&D edition because this is beyond 'house rules' and effectively no longer 5th edition. From the mystifyingly, almost soul-devouring, concept of "needs more MAD" to the brain-draining choices of stat function; it's utterly, inscrutably, totes cray-cray whackadoodle. On a scale of 0 - 10, with 0 being "nope" and 10 being "awesome" I would give this a personal rating of -30.

In summary: It's not really my cup of tea.

At least you're thinking outside the box trying to make things more interesting - I can approve of making that step and starting the journey. Just... Maybe... Not this direction. >.>

The players I know would not make MAD characters with this. They would simply make super-specialized characters with much fewer choices available to them.

It also impacts different classes very differently. Druid is screwed, Rogue is absolutely unchanged.

MAD isn't fun. People always associate uneven stat allocation with minmaxing, but it also assists in roleplaying. People like being the smart one or the strong one, which would seem a lot less apparent when everyone has 14's for everything.