After reading this question on what happens to Sorcerors who lose the ability to cast wish, I started wondering if there are ways for them to regain that ability.
I figured a Divine Intervention is probably strong enough as that is a god's power, as well as a second caster casting wish to undo the first wish (and thereby undoing its effects).
Are these valid ways of regaining the ability to cast wish?
Sadly, I think that's the best answer in this case.
While the use of a Cleric's Divine Intervention or even a Wish may overcome the penalty from the failed Wish, those are both going to ultimately DM decisions. I would agree that either should be sufficient, but if you put me in a corner, I'd say that Divine Intervention is more powerful than another Wish.
Technically, appropriate Divine Intervention is (PHB, 49):
The DM chooses the nature of the intervention; the effect of any cleric spell or cleric domain spell would be appropriate.
Wish is not a valid Cleric spell, nor do any other spells on that list have a means to undo the stress of Wish (although a very loose interpretation of Regenerate may). Allowing the Divine Intervention to relieve the stress would be completely up to the DM.
Using Wish itself would also have to be up to DM as you aren't replicating a known spell. However, you could wish for a certain card of the Deck of Many Things.
So...
Another option that @Adam came up with is The Fates card from The Deck of Many Things is the only true RAW option. It can completely undo that casting of Wish. From the DMG, 164:
The Fates Reality's fabric unravels and spins anew, allowing you to avoid or erase one event as if it never happened. You can use the card's magic as soon as you draw the card, or any other time before you die.
This would effectively turn back time and undo that failed Wish casting that left you stressed.
That being said, the text of Divine Intervention is:
Divine Intervention
Beginning at 10th level, you can call on your deity to intervene on your behalf when your need is great.
Imploring your deity's aid requires you to use your action. Describe the assistance you seek, and roll percentile dice. If you roll a number equal to or lower than your cleric level, your deity intervenes. The DM chooses the nature of the intervention; the effect of any cleric spell or cleric domain spell would be appropriate. If your deity intervenes, you can't use this feature again for 7 days. Otherwise, you can use it again after you finish a Long Rest.
At 20th level, your call for intervention succeeds automatically, no roll required.
The earliest level you can naturally cast Wish as a spell is 17. From a purely mechanical standpoint, it seems a bit overpowered as I wouldn't think a level-10 ability could nullify a rule for a level 9 (minimum player level 17) spell. Divine Intervention also has no real penalty for failure, and the Cleric can try again every time they get a long rest (usually once per day). Statistically, a 10th level Cleric would have an expected value of 53% success [(1-0.1)^7] to let the Sorcerer re-cast Wish if they tried it every day for a week, and that chance only gets higher as they gain levels. At 17th level, they have a 73% chance of removing the restriction after just one week (trying once per day). At 20th level, the chance is 100%, meaning that losing the ability to cast Wish really has no consequences at all (other than the 7-day downtime waiting for Divine Intervention to recharge).
So, if this were to be allowed, then losing the ability to cast Wish would really only stop you for about a week on average (in the worst case) before you could go back and do it again. That seems very powerful to me, and it seems like it almost completely removes the major drawback of a spell as powerful as Wish.
Your question: Are these [Wish or Divine Intervention] valid ways of regaining the ability to cast wish?
Wish (PHB p.288)
You might be able to achieve something beyond the scope of the above examples. State your wish to the DM as precisely as possible. The DM has great latitude in ruling what occurs in such an instance; the greater the wish, the greater the likelihood that something goes wrong. This spell might simply fail, the effect you desire might only be partly achieved, or you might suffer some unforeseen consequence as a result of how you worded the wish
It the DM's decision what occurs if you should decide to try this. The DM does not have to tell you anything about the actual chance of success, only what your character knows or can work out. But YES it is certainly within the power of a Wish to achieve this.
You can Wish for anything, anything at all. What actually happens is up to the DM. It is "only" a 9th level spell still and "the greater the wish, the greater the likelihood that something goes wrong.".
If you Wish for a Demon Lord, Dispater for instance, to die, it is likely that the mere 9th level spell will fail to fully achieve that effect, but might instead give you the chance to kill him yourself... suddenly a Gate appears and Dispater steps out, annoyed that his meal of freshly harvested souls has been interrupted.
Wishing that someone's ability to cast Wish be restored seems reasonable, it is a 9th level spell effect against the effect of a 9th level spell. But be very careful how you word it, the DM will probably not let this just happen easily and the ability to cast Wish is a very great thing and, again, "the greater the wish, the greater the likelihood that something goes wrong."
Also it must be pointed out that this use of the Wish spell may incur (33% chance) the same penalty as the original caster suffered, the loss of the ability to cast Wish, so they had better be a brilliant friend, particularly if the attempt fails and leaves both casters without the ability.
Divine Intervention (PHB p.59)
The DM chooses the nature of the intervention; the effect of any cleric spell or cleric domain spell would be appropriate.
While any cleric spell is appropriate, so may other effects be appropriate, based on your DM's judgement. The wording above does not exclude other effects, just gives a "scale". The DM's judgement should definitely include consideration of the deity's domains and spheres of influence (a deity of fate or time or healing or magic is more likely to intervene in this way than one of evil or war etc), but a deity granting anything a Wish can achieve does not seem unreasonable. If the DM thinks it is appropriate. So YES it is potentially within the power of Divine Intervention to achieve this, it all depends on the DM's judgement.
However a Deity has an agenda so don't be surprised, for instance, if the only Wishes you can subsequently make have to promote that agenda, or that the Deity has power of veto over your Wishes. Don't be surprised if a geas like effect is in place. Or maybe you get an audience with the Deity, where you get to convince them or they bargain with you, convince you, threaten you or trick you into joining "the cause".
