Spell Resistant Feat?

+1 to DEX or CON

Resistance to: Necrotic, Psychic, Radiant, or Thunder damage

d6 to failed save vs. spell or magical effect, Proficiency Bonus times per Long Rest

1) Resistance to Necrotic or Psychic dmg is a great passive.

2) d6 is a modest but solid boost you can use when you need it, against some of the more debilitating attacks from enemies. With so many re-roll / Advantage abilities out there, this gives a straight increase, and of course could be combined with those. Could be an alternative to feats like Resilient or War Caster or other class features, or cold also dovetail and build on those. Scaling with Proficiency Bonus is good design.

3) The DEX or CON boost gives some decent flexibility, since everyone could use more DEX or CON, but may not prioritize it out of the gate. There aren't that many good CON feats that work on a wide variety of builds. So this looks attractive for say a level 12 feat on a martial, or thereabouts.