I have been around different boards, but nothing fits my question. How unbalancing would having another 9th level spell slot be?

As a player, I was planning on taking Wish as a 9th level spell choice for my sorcerer. My GM and I have been discussing how powerful we would like it to be for in game balance. It is one of those spells that can become game breaking because it is so versatile. After a discussion, I was mainly focused on being able to have another 9th level slot. The abilty to cast any spell twice a day of 8th level or lower for a sorcerer could be a massive help if my roll is above 33%. I have Gate as well, so even if it becomes unusable, I have an alternate 9th level spell.

Well... gaining access to a 9th level spell, outside of homebrewing some reason to gain a second 9th level spell slot, can basically only be done by gaining access to a magic item that's capable of casting a 9th level spell. Magic Items of that power are very difficult to come across... any magic item that would allow even a sub-par 9th level spell would essentially be an artifact. The other alternative is crafting a single-use spell scroll... a 9th level spell scroll is considered a Legendary item, and requires 25 workweeks of time and 50,000GP to craft.

So, all things considered... an additional 9th level spell slot is a BIG deal. If it was possible to level past 20, based on just a rough estimate of how long it takes to even gain a second 7th level slot, you would basically have to be at least a level 23 character to get additional 9th level slots... maybe as high as 25.

If you are casting spells then you don't need to roll the 33% chance, that last paragraph of the spell only applies to "The stress of casting this spell to produce any effect other than duplicating another spell weakens you."

So you get to cast one of your 8th level or lower spells using your 8th level slot, and wish for ANY other spell as a 9th level slot.

The act of wishing for an extra 9th level slot would probably cause a big amount of stress. The character is 18th level. Wish would have probably been better served as a legendary scroll, epic boon, or something of the like and just making the SPELL Wish be able to cast any spell of 8th or lower with the spells fantastic bonus of eschewing exotic/pricey components and time.

I just was thinking about having the ability to freely cast with the benefits granted through Wish twice a day instead of once. After looking around and thinking about simulacrum and still being able to drop another 9th level spell in the same day is high on the power list. I have been rethinking the ask, or not asking for anything at all and keeping that kind of request for desperate times or a cinematic event where it plays more into the story instead of just reaching for power.

Which now I am realizing how crazy powerful the cleric's Divine Intervention is, and that is a class feature. Holy cannoli!!!

Another option we were discussing was wishing for an extra attunement slot, but the slot only works for uncommon magic items. An artificer has the ability to gain extra slots for any attunement, so it feels the limitation would be safe power-wise, but still powerful enough to require a wish to get.

Still thinking of an equitable trade-off for a wish that is not too game breaking.

Quote from theHumanChris >>

Still thinking of an equitable trade-off for a wish that is not too game breaking.

 Time better spent thinking about the existing uses for all the other possible 9th level spells which don't require adjudication by the DM as to whether they would break the world? ;-)

I believe there is an Epic Boon that grants a second 9th level spell slot. In other words, you essentially have to have enough XP or hit a milestone at 20th level in order to gain a second one.

The boon of high magic - gain 1 L9 spell slot. The DMG suggests one boon for every 30,000 EXP after 355,000 if using boons as a an “epic” progression.

Thanks, I had forgotten about boons. We discussed it an decided the best use is to transform Wish to Any Spell. Still powerful as is and takes out the chaos of in game balance and other issues which may be unforeseen.

Having played a 20th level character that had the boons for casting an extra 9th level spell and the one that let you cast any spell you know, at any level you have access to, once per day. So my character could cast 3 wishes per day.

3 wishes, plus glyph of warding = insane. 

However, it wasn't that overpowered. The DM was providing epic encounters and even our party of 20th level chars with 3 epic boons each and we still got nearly TPK'd a few times. 

So whether it's OP or not is entirely dependent on your DM. 

Epic Boons, supernatural gift, magic items such as a ring of 3 wishes, and more can provide you with extra 9th level spellcasting.