I've been thinking a bit about resistances lately, and I think I figured out a solution that I like. I'm sure someone else has come up with this before, so apologies if I'm stealing ideas here. Anyway, here's my idea:
Energy Mastery Feats
Energy mastery feats represent a character's specific training with, affinity for, or close bond with a specific type of energy. They offer two benefits. First, they give a damage bonus when you use your chosen damage type.
Second, if a creature resists or is immune to that energy type, you gain an added bonus against it. It's basically the reverse of the idea of giving a monster a boost when you blast it with its favored energy type. So, here's an example:
Dark Soul Devourer
You get a +2 bonus to damage rolls when you inflict necrotic damage.
If you hit a creature and its necrotic resistance or immunity reduces your attack's damage, that creature is dazed until the end of your next turn.
That's not necessarily the exact mechanic I'd use, but it gets to what I'm talking about. My initial idea for this specific feat was to describe someone whose soul was tainted with necrotic energy, and in harming undead (or whatever) they draw the undead creature's life force (such as it is) into their own. But, it's Friday, I'm about to head home, and it's been a long day. You're getting what you paid for!
The Inklings Book of Arthur
1 day ago