An ideia i had sometime, was that change the effectiveness aditivelly. not multiplicativelly:
effectiveness:
+2: +200%= *3 damage
+1: +100%= *2 damage
+0: +0%
not effectiveness:
-0: -0%(1/1)
-1: -50%(1/2)
-2: -67%(1/3)
That way, the maximum damage of SR would be only 37,5% rather than 50%, and the minimum damage would increase(3.something% to 4.something%)
But, thunderbolt would cause only 3 times the damage in a gyarados, rather than 4.
And the maximum resistance would be 1/3 rather than 1/4.
What i think that is a good thing: who dies easy for a weakeness would survive more, who lives easy for a resistance would die easier...
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the mega articuno ability could be magic guard.