We could test no immunes on PTR

Totally agree

This is the substitute they should test in PTR

No more immunities, but monsters with high resistances are super hard

Immunities are a bad global design

On top of this, a lot of Resistance type disparity could be solved with the Hell% mechanic

This mechanic makes the remaining monsters more rewarding yet harder the less of them are remaining in the game. They get a buff to no loot, dmg, xp, life, mlvl and Resistances including magic resistances

The Buffing monsters magic resistance in the mechanic would be a great equalizer