I can see Toughness helping against being Stunned, not sure how tough you are should count for your balance or stop you from having your leg swept. I am not sure tying total immunity to a stat would be good, since you can just drink some Spiced Wine from Garth or Vitra and just shoot it up to about 23. Which means immunity would either be within easy reach any time you could get a drink (which is almost anytime before any fight or dungeon), or it would have to be so high that it would be near impossible unless you focused everything on Toughness.
Probably best to keep it as a skill benefit, like Equilibrium.
first off, i imagine as someone trying to leg-sweep or charge the champion but the force wasnt not enough to knock them down because the champion is a literal immovable object, could even have silly mode interactions too
just want to add that this doesnt mean they are immune to the damage of the attack
second, make that alcohol buffs not give the resistance/immunity or less of it, which is probably a requirement because theres a consumable you can carry, to max out toughtness at any time... so yeah, you are correct and it would been more broken than you thought
and finally, i dont think it should be exclusive to Equilibrium and there should be more ways to counter Stun/Prone, it doesnt need to be everywhere or easy use or with no cost
so a Encounter or Recharge power that gave stun immunity for that fight or X turns
a conditional passive with requirements not everyone would want to deal with
or even just a item with drawbacks too
i just tired always using equilibrium, wishing i could choose other At-Will powers
wishing could sacrifice other slots for the effect, because not using equilibrium also sucks