Considering adding this to my game. Be interested in your feedback

Heroic Effort.

As a reaction after failing an ability check or attack roll the character can make a Heroic Effort. This has the same effect as the 2nd level fighter ability Action Surge. After the additional action, the character takes on a level of Exhaustion. 

Rationale. 

It can be frustrating as a player to try something important and have it fail, this would give a character one more chance. Especially useful for those last-ditch efforts. 

Balance

Limited occasions, only work after you've failed a roll. Also, since it uses your reaction, it can only be once a turn. 

The consequence of Exhaustion is pretty self-limiting. Using it more than once between rests would be pretty crazy unless you knew this was the end of the adventure.

Quote from Malzerious >>

Considering adding this to my game. Be interested in your feedback

Heroic Effort.

As a reaction after failing an ability check or attack roll the character can make a Heroic Effort. This has the same effect as the 2nd level fighter ability Action Surge. After the additional action, the character takes on a level of Exhaustion. 

Rationale. 

It can be frustrating as a player to try something important and have it fail, this would give a character one more chance. Especially useful for those last-ditch efforts. 

Balance

Limited occasions, only work after you've failed a roll. Also, since it uses your reaction, it can only be once a turn. 

The consequence of Exhaustion is pretty self-limiting. Using it more than once between rests would be pretty crazy unless you knew this was the end of the adventure.

the consequence is only self limiting to the point where players know/have access to greater restoration. After that point. it's basically you can trade a 5th level spell slot for the extra chance.

given that it is a "heroic" effort... and to discourage the greater restoration/always immediately before a long rest, ways to abuse it. Maybe tweak it slightly to where It works like a "legendary resistance" in that it turns the failure into a success no matter what. But make it give 2 or even 3 levels of exhaustion instead of 1. 

^ in this manner, players can "heroically" pull something off they should not have been able to do ordinarily, but with the 2 or 3 levels of exhaustion, the following after, and next day, they're still feeling the effects. Or, it will at least force enough use of resources to immediately fix, that it discourages it from being an overdone thing by everyone in the party every session

My rationale for the above:

It can be equally frustrating as a player, when your character has a specific purpose design, and your character is made moot or redundant because *anyone* can just do the thing you tried to make your specialty.

Quote from EightPackKilla >>

My rationale for the above:

It can be equally frustrating as a player, when your character has a specific purpose design, and your character is made moot or redundant because *anyone* can just do the thing you tried to make your specialty.

This is a functional problem of all the classes and subclasses in the game. They lack a Defining Trait in the sense of a specific mechanic that can't be copied among any other class or subclass. There is reliance on the archetype as the thing that distinguishes them, but when the same archetype exists among all the different groups of classes, it renders them bland, and takes away what make the archetype an archetype in the mechanics of the game itself.

I confess, it is a design choice that irks me, but it is part of the structure as intended and permitted (going by all the homebrew classes and subclasses). That is, it is intended for people to be able to do something like that regardless of class as a functional part of the game's design.

The mechanic you propose as a whole is fine if it works within the game being played. If it is a feat, it would be a feat available to everyone. If it is a general action, it owuld have to be available to everyone. If it is a class function, then until someone else creates a subclass and uses it, it would be fine.

It does, however, have a risk of breaking the skill set up as it is, since it allows for use after every failure, and the exhaustion application only discourages its use, not stops it until the character is wiped out.

And the thought of knocking yourself out cracking open a safe is a bit much, but fits the larger schema.

Better would be to place an additional limit on times per day or it stops being heroic, and starts being desperation.