I'll be quite frank: this is a completely stupid idea. It isn't "immersive", it definitely isn't fun, and it also doesn't logically follow / make sense.
I'll explain the issues stat-by-stat, but I'll have to paraphrase the OP because it's a bit awkwardly worded.
STR: When STR is over 15, NPCs and peaceful animals become naturally uncomfortable with your presence.
Yeah, that's what Charisma is for. A strong guy with high Charisma is a big sweet papa bear. A strong guy with low Charisma is a swearing, drunk, brawling oaf.
DEX: When DEX is over 15, allied NPCs can't keep up, and you become susceptible to Criticals from blind spots due to your arrogance
Like before, another stat more intuitively handles Criticals: Perception.
Criticals are not the antithesis or opposite of a clean miss, and it makes no sense that "arrogance" courtesy of pro dodging strats would allow you to keep dodging every single hit except Criticals. Criticals are a simplified amalgam of blind spots (DEX & PER), weak points in armor or anatomy (CON), approach to perfection of attack v. dodge technique (INT), lucky or skilled timing (DEX & PER), and just plain hitting way damn harder than normal (STR), alongside other discrete factors that can't be paid the proper individual attention in a Diablo- or MOBA-style MMO context.
Also, you're basically saying to cap the movement speed buff at the movement speed of your slowest party member, since you'll be stopping for them to catch up, making a sufficiently high movement speed buff largely pointless outside of close combat within a small area.
Screw that, I've had enough of that mess from Secret of Mana.
INT: When INT is over 15, you speak arrogantly to NPCs, and your attunement to mana decreases your resistance to magical attacks.
Yo. Yo. Charisma exists. Remember that stat? That determines whether you rub people the wrong way or not. That's the express and sole definition of the stat.
But gosh, what a great idea: remove the only form of defense a classical INT character has. In every single other RPG in history, INT increases your magic defense. You know why? Because...
- No other stat does, since we don't have "WIL" or another dedicated magic resist stat. CON is the physical resist stat and if it was also the magic resist stat then CON characters would be invincible which is not acceptable.
- INT-heavy characters sacrifice some form of physical prowess, either in STR, CON, or both, so it's only fair that they get magic resistance since they usually won't have physical resistance
- Because if INT is understood implicitly as the "knowing how magic works" stat, hence why a high-INT character casts better / stronger spells, then it should also be the "defending against magic / counter-spelling" stat
CON: When CON is above 15, healing & buff potions are less effective alongside poisons.
A higher body mass increases the dosage needed for any drug, poison, etc. to have the same observed effect, but Constitution is not body mass. Constitution is fitness and health. Here in real life, a healthier and more fit person is more resistant to disease and better-affected by positive medications and other medical treatments compared to an ill person.
That is because poisons work against the body's systems while medications work alongside them. This is a necessary element of the intelligent design of a poison or medication.
Consequently, the stronger said body systems are (Constitution), the less effective poisons will be and the more effective medications will be.
PER: When PER is above 15, you become suspicious of everyone around you.
? ? ? Why exactly?? It's called Perception, not Paranoia. Perception would make you less suspicious of (innocent) people around you because you were better able to perceive their true intentions and thus know in reality whether they were innocent or not.
CHA: When CHA is over 15, you begin attracting bad attention and monsters aggro to you from further away.
So, high Charisma does what low Charisma should do. Got it.
What do you think the point of the stat is? It's social awareness and the ability to doctor your behavior to get the desired response /attention.
You seem to be mixing it up with Fame.