I've created a lot of content for 5e in this blog, but only a tiny number of feats. That's because feat design is one of the hardest things to do well in this edition: far from the wild and woolly, anything-goes atmosphere of 3.x and 4e feats, WotC's feat list is scanty, but each feat has several moving pieces that fit together precisely. Of the feats in the Player's Handbook, only five directly involve spellcasting (Elemental Adept, Magic Initiate, Ritual Caster, Spell Sniper, War Caster), and two of those are "entry level" to being a spellcaster in the first place (Magic Initiate, Ritual Caster). To that end, I set out to write a few more feats to support spellcasting.
If you're thinking to yourself, "But I like the very short feat list - I think that's one of the game's strengths," take a deep breath and realize that this is a design blog. This feat list could have been inscribed on tablets of purest gold and handed to Mearls and Crawford by the Archangel Gabriel, accompanied by a really kickin' horn section, and I would still poke at it to figure out if I could contribute anything to the ideas therein.
Also realize that one of the most common responses to "what kind of content do you want to see in 5e?" is more feats. Anything I make here needs to pass a high bar of interest, balance, and theme; it needs to be something you can see in the fiction, not a few bonus points to a particular kind of attack or check. If there's a limiting mechanic, that limiter should make sense in its context. There can't be too much data-tracking to do, and it can't totally hose the action economy. This is why I say feat design is hard.
Prerequisite: Able to cast at least one spell
Thanks to rigorous discipline in practicing magic, you gain the following benefits.
When you cast a spell that restores hit points to an ally, you gain temporary hit points equal to the highest die value that you roll. These temporary hit points last for up to 1 hour.
Gain advantage when you make a Constitution saving throw to maintain Concentration on a beneficial effect you have cast on yourself or an ally.
When you fail a Constitution saving throw to maintain Concentration, you may choose to gain a level of exhaustion and treat the saving throw as a success instead. All exhaustion gained in this way is removed with a short rest.
Prerequisite: Able to cast at least one spell
Thanks to ruthless dedication to the ways of magical power, no matter the costs, you gain the following benefits.
When you roll damage for a spell you cast, you may choose to gain a level of exhaustion to reroll a number of dice equal to your proficiency bonus. You must keep the new result. All exhaustion gained in this way is removed with a short rest.
Gain advantage when you make a Constitution saving throw to maintain Concentration on an effect you have cast on yourself or an enemy.
When you cast a spell of 2nd level or higher and all creatures in its area or that it targets are immune to its effects, you regain an expended spell slot. That spell slot must be of a level lower than the spell you cast. If you used Pact Magic to cast the spell, your Slot Level for that Pact Magic spell slot decreases by 1 until you complete a short or long rest.
I am not extraordinarily attached to the names of these feats, but I started with what sounded like striking names and tried to build compelling features from there. To my eye, Mystic Clarity stays more on-message than Mystic Corruption does. In principle, I would allow a PC to flip back and forth between these two feats with a Recuperate downtime action (ritual purification or dissolution), but in reality Mystic Clarity is useful to a smaller set of all characters than Mystic Corruption is, since most arcane casters have no means of healing anyone.
Mystic Clarity Breakdown
The temporary hit points feature of Mystic Clarity comes out of liking how Fighting Styles and weapon-wielding feats (and Elemental Adept) handle the modifiers that they throw out - things like the Great Weapon Fighting style show us more direct engagement with the value on the die. (I would cite 13th Age as a case of pushing that concept quite a lot further, and the One Roll Engine as its logical extreme.) Anyway, having a feat give you between 1 and 8 temporary hit points (since the overwhelming majority of healing spells use d4s or d8s), and heal/mass heal get nothing, is more or less okay with me; if those things bother you, change it to 2 temporary hit points per level of the spell slot expended.
The second feature is a shot across War Caster's bow. In my gaming groups, players with any spellcasting power are likely to treat War Caster as their nearly-obligatory first feat selection. There are players who don't do this, but they're usually making a conscious uphill decision. I wanted, therefore, to grant a piece of one of the more crucial War Caster functions, so that you might look at this feat as a viable alternative; likewise for Mystic Corruption's second feature (but I'll come back to that). If you take this feat, you're making a series of statements about how you plan to play, and that style is very friendly to bards and clerics.
Thinking about it right now, as opposed to when I wrote these feats two days ago, I've added "yourself or" to both Mystic Clarity and Mystic Corruption's second features. This makes an immense difference, as it clarifies how both feats interact with the general category of alternate-at-will spells: moonbeam, call lightning, vampiric touch, et al. It shifts the flavor of Mystic Corruption to be slightly less about aggressive play and slightly more about aggressive or selfish play. (Also, casting defensive buffs on yourself is always a dubious proposition; if you're enjoying the benefit of stoneskin cast on yourself, you might also be losing the spell now.) I also see that what these two features don't clarify is which of them, if either, apply to conjure spells. Defining the "target" of a conjure spell is not perfectly intuitive. There's the obvious answer (the summoned creature is the target), but the creature is necessarily not within range when the spell is cast - the actual target is a point in space, to which the conjured creature is in no way bound. Therefore, such spells don't interact with either feat.
Moving on. The third feature is where the clarity part really comes in - you're pushing yourself beyond normal limits, sacrificing the physique to protect the mind. You can convert exhaustion into sustained Concentration. Because exhaustion past 1 is really, really bad for you, and again clarity as a concept, you can dump all exhaustion gained in this way on a short rest (presumably some meditation). Needing to go to a second level of exhaustion before you've taken a short rest won't come up all that often, but when it does, maybe this makes you feel cool because you've gotten to do it. This came about because I was playing when benefits when you succeed Concentration checks and reduced bad outcomes when you fail them, as a design space. That led to flipping failure to success-at-a-cost, and then looking for an appropriate cost.
Mystic Corruption Breakdown
Right off the bat, you see that I'm playing with dice roll results and re-using exhaustion as a cost mechanic. This is something you're a lot more likely to use multiple times between short rests. It may be irony (or just "this only makes sense in my own head") that I've used similar mechanics as the central thematic element of both clarity and corruption, but to me, sacrificing the physique for increased damage shows a vengefulness and kind of internal decay that fits. Sacrificing the self for primarily protective magic on one's allies fits with what I mean by Mystic Clarity.
See above for notes on Mystic Corruption's second feature - most of the same points apply. This feature makes this feat very appealing to druids, wizards, and warlocks. Both feats apply to a paladin's smite spells, which is an important outcome to me.
The third feature is another attempt to mitigate the evils of so many immunities (both to damage types and conditions) floating around in monster design. You reclaim some of the lost power from a spell that never could have done anything, which I hope makes you more comfortable experimenting with damage types and conditions for which immunity is more common. You still lose your action, but I can't solve that short of straight-out turning off a creature's immunity, which I don't want to do because it is part of a creature's intended power and theme. The step-down mechanic is a riff on the Diviner wizard's Expert Divination feature, plus handling for Pact Magic.