Khemsa's girdle

A magic item drawn from Robert e Howard's "people of the black circle", Khemsa's girdle is braided out of thick hair that appears to be human interwoven with small bits of jewels. I haven't quite figured out attunement for it yet, perhaps it only attunes to chaotic characters á la Conan. The girdle grants its wearer advantage on saves vs illusion magic but also has  the chance of deflecting damage onto nearby allies in combat situations. The damage deflection activates on a 5 or 6 of a d6 and will apply the damage otherwise inflicted upon the wearer to an adjacent combatant whether friend or foe. In the event that there are multiple combatants adjacent the target may choose the direction of the deflection but may not choose to forego the effect to avoid injury to allies.

Exhausting magic

I think one of the most intriguing mechanical elements of 5e is the "levels of exhaustion". I have yet to use it in game but I think it would make a very interesting story driver/consequence engine. It adds granularity to damage in a way that hit points don't. Something else I have been pondering is how one might add some grittiness to the magic system and I came up with a way to showcase levels of exhaustion while adding some risk to using magic. taking inspiration from the wild magic of the sorcerer class, what if whenever a magic user had a crit fail he or she gained a level of exhaustion? Now a lot of magic doesn't use a direct attack roll and is just save versus spell, in which case the exhausting magic might not apply. alternatively if the target of the spell Crits their save maybe the spell bounces back up on the caster? These house rules might go nicely in a higher danger setting with the permanent disability rule in effect. The key would be finding a balance where the added risks don't make the lethality unplayable. Perhaps it would be fun to try in a higher level one-shot where the power balance is purported to swing in the players favour. My literary proof text for this idea, btw, is in A Wizard of Earthsea when Ged over exerts his ability and is blinded for several weeks.

Fortune cookie

Yesterday I challenged my partner to make a fortune cookie equivalent for my world. She knocked the ball out of the park I think, her idea is as follows: some sort of snobbish wizard decides that their work is too good for posterity and buries a number of books from their library. Perhaps they even shred the work before throwing it in the hole. By some strange magical accident a tree grows up from the Book hole and snippets of text are found in each of the nuts that grow upon it. Either this can be a bit of quirky flavor in the world or it can be used as a story hook. Perhaps years later, long after the wizard is gone people use the tree as a divination tool taking advice from the nuts. Maybe a whole religion grows up around it, a monastery is built around it and monks guard it's sacred nuts.