Resistances & Immunities Aren't Any Fun.
This may be controversial! You may not agree with me.
You don't have to!
But as a player, I've found that my most frustrated moments when it comes to a session are the times that I can't do anything because the enemy is resistant or immune to what I'm doing and I don't have a lot of alternate options. And resistances and immunities - well, the stats have been told a thousand times before; it's usually non-magical melee, and I have been playing characters with non-magical melee weapons. Let's face it: melees absolutely get the short end of the stick when it comes to resistances and immunity mechanics. And it's boring for both players and DMs when something hits and absolutely nothing happens or you just get muted damage, along with the woes of feeling like you're letting your team down.
Despite my grumbles, I started thinking about this issue from a DM standpoint as well. Rather than having resistances or immunities, what else could happen when a player hits an enemy with a certain damage type that could be more interesting, fun and unpredictable to keep them engaged?
No More Immunity: Homebrewed Alternatives to Creature Resistances & Immunities is a document with 12 suggested alternatives to simply having a monster be resistant or immune to a certain damage type.
- from the publisher's blurb