D&D 5E Cost of make a Saving Throw

I did not want to derail @Hemlock's thread so I created a new one. In Helmock's explanations for what he would miss from 5e if he instead played AD&D he touched on something I thought was worth a discussion.

Hemlock said:

...(snip)...why exactly a Dexterity saving throw to protect you from Fireball does not cost movement or a reaction or leave you prone (i.e. apparently you're not doing any of the things you'd think you were doing to protect yourself from the Fireball).

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What would be the implications for introducing a cost in order to make a Dexterity saving throw?

Cost could be anywhere from a loss of a Reaction or suffer the Prone condition or a loss of future movement or something else entirely.

Personally I'm not a fan of the loss of movement just because it is something you have to remember upon your next turn, though you could argue the Prone condition is also something you'd have to remember, but I feel a condition is easier to keep record of.

The issue some might have with using a loss of a Reaction or Prone condition is that if you have already used your Reaction then you can only save by taking the Prone condition. What if you have already used up your Reaction and, Mystra forbid, you are hit with 2 Fireballs before your next turn - do you not save on the 2nd Fireball or do you make a save with Disadvantage?

The other question one could ask is why would this mechanic only be applicable to Dexterity saving throws? Wouldn't mind-affecting spells also require one's attention? i.e. therefore a Reaction cost.

Perhaps this all is making things unnecessarily complicated, perhaps 'movement to resist affects' is a free reaction within combat with no limitation to how many times one can react to resist.

I think that if you're going to go this way, you may as well kick out Dexterity saves altogether, and replace them with something you can use more consistently. For instance, to resist a fireball you could choose to either make an Athletics check to move up to half your speed or an Acrobatics check to duck into cover, with either choice using your reaction.

It does feel like it'd be a lot of work just to patch up a minor conceptual issue, but I guess it depends upon just how bothersome an issue it is for you and your group.

Well, for starters, you would have to figure out why avoiding a fireball with a DEX based save has a cost while avoiding a sword with DEX based ac doesn't.

The cost/implication of this idea would be needless complexity.

I think that you have pointed out the worst of the problems yourself, the next question to ask is "what does it bring to the game?" Personally I don't see a benefit to playing from doing this. It only covers over a small simulationist oddity that appears some of the time and it raises more questions, why do you get to make a strength save without penalty, should that not require your bonus action to brace yourself by adjusting your footing and leaning into the cause of the save? What about intelligence saves? Should that not take your free item interaction to pull out your pen and chew the lid?

To use Hemlocks example of a save vs fireball, think of it more as you react fast enough to turn your back on the blast (half damage) or you don't and catch the blast full in the face (full damage). Arguably a fireball should send everyone flying away from the blast and, especially in a 10x10 corridor, gasping for breath from the oxygen depletion.

A potential way to include an aspect of a 'cost' is to make it player-optional and grant some form of bonus to the save if the PC is placed in a disadvantageous situation of the DM's whim (prone, used X feet of movement from next round, blown reaction, whatever). Advantage is probably too much; perhaps adding an extra proficiency modifier granted on the save in question. You could do similar effects with the other saves for different disadvantages -- stunned until next turn might be an option for mental saves, for example.

What would be the implications for introducing a cost in order to make a Dexterity saving throw?.......
Why in the H double E double L do we want to introduce a cost of any SAVING THROW. Well except of Morrus who takes 3 minutes to find his lucky d20. In 1E we stood next to people who failed their save and had Helm's of brilliance cook off. After lots of saves and taking lots of HP damage, we just swept Jr's ashes into the helm, and called dibs on his boots of Elvenkind.

I think of STs as "twitch" reactions. You don't think about or plan them, they're instinctive and lightning quick. Example; one time my old boss was talking to me at her desk. She motioned with her arm and knocked a glass off the desk. Without missing a beat she grabbed the glass as it fell without spilling a drop while carrying on with the conversation.
They're one of those things that happen in a split second and succeed by a whisker so that bystanders go "Woah!"

In Savage Worlds, when you are targeted by an AoE like that you make an Agi-2 roll. If you succeed, you move yout characters outside but adjacent to the area and take no damage. Fail and you take it full. No action penalty.

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Hmmm... if I were to do something like this, I'd borrow from Fate (kinda). You make saving throws as usual, but if you fail, you can opt to take take a complication/penalty effect. So...

"You failed your save against the fireball. Take 35 damage *or* your lose your armor because it melting."
"You failed the save against mind control. You can opt to resist the effect, the strain will mess up your vision center & leave you blind."

If I did this, I'd throw a lot more save causing situation at my players. I'd also reserve the "take a consequence" option for dramatic moments. I like the idea of giving a player the choice of turning one problem into another problem.