What would you change in Heroes of Might and Magic IV?

....take, for example, Hypnotize or Berserk from Haven. If there's no resistance to Haven, it will work 100%...

Have you tried increasing magic resistance? Just the regular kind? This is for heroes; a cloak with resistance would work for the army units.
It worked for me, maybe not 100%, but the spells didn't trigger. Arandor can confirm; last year, we played a tournament game: he was Haven, and I was Sylvan. In the final, he tried very hard to cast various nasty spells on me, but it didn't work.