I'll be the DM for a new campaign and I don't particularly like how Impervious was handled in this edition. But, I also think that the 2nd edition one was OP (overpowered). The idea behind the solution is to make the defence with impervious less likely to be damaged by an attack, but still has a chance to damage it with a "lucky" shot.
(note: Since Penetrating is the counterpart of Impervious, it did little sense to me to divide this question into two questions.)
I added, in italics, the following.
Any effect with a resistance difficulty modifier equal to or less than half the Impervious rank (rounded up) has no effect. Furthermore, if the resistance difficulty modifier is equal or less than the impervious rank, the defence roll has advantage (roll two d20 and take the highest). The ranks in impervious cannot surpass the ranks in the the defence that it affects.
For example, if a resistance difficulty modifier is 9 and the impervious rank is 10, the defence roll would have advantage.
...the target must make a resistance check against an effect rank equal to your Penetrating rank. The defence check against an attack with Penetrating is made normally (without advantage) if the ranks in Penetrating are at least half the ranks than the Impervious ranks (rounded up).
Example, an attack 10 with penetrating 5 would ignore the advantage of a defence with impervious 10 but not of one with 11. Moreover, an attack 4 with Penetrating 4 will force a defence save with advantage against 4 damage.
Are these changes to impervious and penetrating OP?