I was a big fan of 3.5.  There, I said it.  Some of you will hate me for it, but I did.  What I liked was customization.  I enjoyed how mechanically different things could be.  I say this to lead into the topic of the Feat: Energy Substitution.  Here is a link to the text and I have reproduced it here:

Energy Substitution [Metamagic]

Prerequisites

Any other metamagic feat, Knowledge (arcana) 5 ranks.

Benefit

You choose one type of energy: acid, cold, electricity, fire, or sonic. When employing a spell with the acid, cold, electricity, fire, or sonic designator, you can modify the spell to use your chosen type of energy instead. The altered spell uses a spell slot of the spell’s normal level.

The altered spell works normally in all respects except the type of damage dealt.

Special

You can gain this feat multiple times. Each time the feat applies to a different type of energy.

Now I am ramping up to run Princes of the Apocalypse with my group which includes people I've played with for a couple decades.  It occurred to me that I would like this feature available to all spellcasters in 5e in my game.  However I had not thought about making my own feat, that is until I got on here and started to tinker.  It seems like a fun challenge and heck, if I am happy with it I can release it to others (since I don't see it yet).  So I have read the DMG and the feat design article, and wanted to toss out what I have here and see what people think.

Prerequisite: Must be able to cast spells of first level or higher.

 

You are practiced in weaving a specific elemental type into your spells.  Furthermore, as your skills with this element have been honed, you may use your magical strength to blunt that element from hurting you.

When you select this feat, select one of the elemental damage types of: acid, cold, fire, lightning, or thunder

This feat may be taken multiple times allowing for a different elemental type each time.

Then you select one of the five elemental types which have the following text for their option. (Here Acid is replaced by the chosen Element)

1. Spells you cast which do a damage type of acid, cold, fire, lightning, or thunder may instead do Acid damage.
2. When you take damage from a source that does Acid damage, you may, as a reaction, spend a spell slot to reduce the damage by an amount equal to your primary spellcasting ability modifier multiplied by the level of the spell slot spent.  In the case of a Warlock, the slot spent counts as the highest level spell you can cast from the Warlock class.

I added the energy reduction because just damage type seemed too bland.  I was thinking of making it part of a tree of Elemental Mastery feats but combining this made it interesting to me.  Here is where I am asking others about their tastes and for constructive feedback.  What initially turns you off about this feat as a player?  How about as a DM?  Conversely is it something you would want to take or use in your games and characters?

I think it's a fine feat. Consider making the prerequisite "The ability to cast at least one spell" as this is standard for other spellcasting feats. I would add poison damage and rewrite the second ability to sound like

• As a reaction when you take the damage of the chosen type, you can expend a spell slot to reduce the damage by an amount equal to your spellcasting modifier times the expended spell slot level.

Quote from Astromancer >>

I think it's a fine feat. Consider making the prerequisite "The ability to cast at least one spell" as this is standard for other spellcasting feats. I would add poison damage and rewrite the second ability to sound like

• As a reaction when you take the damage of the chosen type, you can expend a spell slot to reduce the damage by an amount equal to your spellcasting modifier times the expended spell slot level.

Poison typically falls into the same category as Fire and Cold as many monsters have resistance or immunity to poison and many weapons or attacks deal poison damage.

Ah interesting.  I was basing it off the Elemental Adept feat as a model since that was most similar to the 3.5 version of the feat

So I submitted this as 5 feats like the Elemental Adept feat is represented.

I was rejected:

This is not upsetting, because it is a sensible reason.  So I have a version that I made which made sense.  However what I cannot figure out is how to allow a character to take the feat multiple times.  I cannot seem to find a setting for that.  I assume there is not a way to do that and that is why Elemental Adept is set up as five individual feats.  Any thoughts?

If not, I might adjust the feat to be more potent but also only allow it to be taken once.