I like where a lot of this is going. From what I'm reading here is that there are a few things the community as a whole is seeking.
1) Class specialization and/or identity.
2) Stat utilization or specification.
3) Ease of adoption for New Players.
4) Ease of adaptation for Veteran Players & DMs.
5) Ease of implementation through a Class Card Reprint.
As someone comparatively new to true dungeon veterancy, my recommendation is to maintain the status quo for what Strength, Dexterity and Constitution offer for and utility. This allows for us to satisfy conditions 2, 3, & 4, and is less work overall for a rework.
That leaves room for specificity & growth for Intelligence, Wisdom and Charisma to be adjusted.
Focus currently exists as a catch all for spell damage, healing output, and polymorph damage. Leaving this to Wisdom satisfies conditions 2, 3, & 4.
In terms of gameplay and roleplay, the most satisfying experience comes with interacting with rooms and NPCs. So on paper, Intelligence and Charisma can be attributed for different things, since we're using our natural charisma and observational intelligence to solve puzzles.
Currently the Charisma state functionally operates as a slot expander for figurines. Totally fine imo. This satisfies conditions 2, 3, & 4.
The most underleveraged stat in TD is Intelligence. This is where I see the most room for growth and class specification. In terms of gameplay, as previously stated, we as players are already using our observational intelligence to resolve puzzles. So what does that mean for our characters? I think this is where we can focus our energy for stat adjustments.
If Intelligence becomes the class specialization stat, then we can keep our eyes on how Intelligence boosts the class at all.
For example:
If we want Clerics to outperform Druids on healing, then Cleric as a class uses its intelligence modifier to increase its healing output. Flavor wise this is a connection to the divine.
If we want Rangers to outperform martial classes on to-hit bonuses via accuracy, then Intelligence modifier to increase its to-hit output. Flavor wise this is a connection to hitting far reaching targets
If we want Elf wizard to outperform Wizard on high damage, then the Intelligence modifier to increase its critical range. Flavor wise this is a different vector for multiplanar energy shifts.
If we want Bard casting or Bardsong to be boosted etc...
If we want Monk AC to act differently, use Intelligence modifier to increase AC.
I believe with that, we can design things in such a way that we satisfy points 1-5. And it allows us to think more about how each class would leverage their intelligence and scale it appropriately. As is printed on a relic level ruling "(increased Intelligence does not have a direct in-game benefit, but some tokens have a minimum Intelligence requirement which this token could help fulfill)" I think that's where we have room for play without massive reprints or rethinks.
tl;dr: Keep all other stats as they are, leverage intelligence to be class specific.