Take a Downside to Solve the Accomplishing Nothing Problem

The Accomplishing Nothing Problem:

• If you are in combat and get hit by an effect that temporarily takes you out of the fight and you fail your save, you are about to spend an amount of real-world time potentially measured in hours doing nothing. That's a recipe for player disengagement at best.
• If you cast a high-level spell and your target makes their save, not only have you wasted your Action, you have blown one of your main limited daily resource, and accomplished nothing to show for it.

The Swedish TTRPG Dragonbane has a solution where, if you fail a roll, you can take a bane to one of your abilities to reroll.

I've been rolling around in the back of my mind, and trying to remember, the possibility of offering a choice of minor consequence to turn an almost-success into a success.

Then, recently, I had a burst of inspiration in the shower, and hacked together some actual numbers and stuff. The main downside is it may be too complicated, though I laid it out in a way that flows fairly well, at least for my mind.

To wit:

If a d20 test is attempted by…

1. you (a Significant Character: any PC or most named NPCs)
2. a character you are Helping (or otherwise giving a bonus to, eg with Bardic Inspiration)
3. an opponent, producing an effect that would affect (1) or (2)
4. an opponent, resisting an effect produced by (1) or (2)

…and (1) or (2)’s test would narrowly be a failure, or (3) or (4)’s test would narrowly be a success:

• Let D = the amount the d20 roll would have to differ to make the failure a success, or the success a failure.

If D is less than or equal to your proficiency modifier, then you have the option to:

• spend a number of Hit Dice equal to D

and

• take a Consequence (by rolling on the Consequences table)

and

• turn the failure into a success or turn the success into a failure

Consequences Table
Roll	Consequence	Details
1	roll again twice, taking both results
2-5	an item breaks	an item that makes sense in context (eg, weapon used in an attack, focus or material component used in a spell); consumables destroyed; nonconsumable magic items have magical power suppressed but they come back if item is mundanely repaired
6-11	all foes have advantage to attack you	until the start of your next turn
12-14	you are Blinded	goes away with Short Rest
15-17	you are Deafened	goes away with Short Rest
18-20	you are Frightened	goes away with Short Rest or if the thing you're Frightened of is destroyed
21-26	your next d20 Test is at Disadvantage	goes away if you don't make a d20 test before your next Short Rest
27-33	you take a Strength Bane
34-40	you take a Dexterity Bane
41-47	you take a Constitution Bane
48-54	you take an Intelligence Bane
55-61	you take a Wisdom Bane
62-68	you take a Charisma Bane
69-76	you take Dd12 damage	damage is of a type that makes sense in the context
77-81	you take a level of Exhaustion
82-89	you provoke an attack of opportunity
90-94	you drop an item	an item that makes sense in context (eg, weapon used in an attack, focus or material component used in a spell)
95-99	you are knocked D squares backwards and prone
100	you take an Ability Bane of your choice

(An Ability Bane gives you Disadvantage on all rolls that use the Baned ability. When you take a Short Rest or Long Rest, you can spend Hit Dice to make one Ability Bane per Hit Die spent go away.)