House-rule for letting high-level casters invent their own spells

Hi! I'm trying to come up with rules that allow PCs to invent new spells, while making it clear that it's something only the best spellcasters can do, and that it's very risky even for them.
I could use some feedback :)

Creating a new spell is an extremely difficult and dangerous endeavor. Discuss the new spell with your DM first and make sure they approve of it. You must have a spellcasting feature and proficiency in Arcana. You must also have spell slots of the level of the spell you are trying to create, and it must be on your spell list.
As you work on your new spell, you must spend the same amount of time and gold normally required for a Wizard to transcribe a spell that they don't know into a spellbook, with a minimum of 8 hours. You also need 8 more hours of work if you are not a Wizard. You cannot work more than 8 hours per day, and the work's duration cannot be shortened in any way. For each day of work, roll an Arcana check (DC 20 + the spell's level). You can use the help of an assistant who also meets the requirements to give yourself Advantage on the roll.

On a fail, a mishap will occur according to the spell's associated school of magic, and the day's efforts are ruined. If you roll a natural 1 on the Arcana check, the mishap is catastrophic, and both you and your assistant (if you have one) die. Once you have accumulated enough successes, you know the spell and it appears in your spellbook if you are a Wizard. If you are any other spellcasting class, you can replace one of your known spells with the newly created one over the course of a Long Rest. If your assistant helped you every day while creating the spell, they learn it too.

Possible mishaps:
Abjuration: "You lose your ability to do magic until you take a Long Rest."
Conjuration: "A swarm of small monsters constantly appears and wreaks havoc around you for the rest of the day."
Divination: "You mind is filled with incomprehensible predictions of infinite future possibilities until you take a Long Rest."
Enchantment: "All of your memories since you last woke up are wiped."
Evocation: "An explosion of your spell's element occurs, injuring you and destroying your day of work."
Illusion: "You fall into a deep coma until the next day."
Necromancy: "You become sick and vomit uncontrollably until you take a Long Rest."
Transmutation: "Your body becomes painfully distorted and you are horribly disfigured, making you unable to work. Your form returns to normal after a Long Rest."

I don't think the risk of death will dissuade many, especially with a Cleric in the party, and especially if they can roll with Advantage. But it sets the tone.

I'd start by setting the minimum time and cost requirements to at least the cost of creating a spell scroll, not just scribing one into your spellbook. If you're looking to be consistent with existing ideas around crafting new spells, I think either the 3.5 DMG or the 3.5 Wizard splat book, Tome and Blood, had the last published official rules on creating new spells. Obviously, you'd want to retrofit the rules a little bit, since 3.5 and 5.5 are WILDLY different rulesets, but it might offer some insights into a direction to go.

Otherwise, if this fits your in-game narrative... I mean, the DC is pretty high. A level 20 Wizard, even with expertise, would typically cap out at +17 to an arcana check - MAYBE a little higher if your campaign is magic-item-heavy. I might recommend having the DM set the DC depending on a series of criteria, most notably if the spell is entirely original, or just a light modification of an existing spell - for instance, a Fireball that deals Lightning damage might require a lower DC if your spellcaster already knows Fireball, vs. crafting an entirely original spell with an entirely original effect.

Quote from RWinnie >>

I'd start by setting the minimum time and cost requirements to at least the cost of creating a spell scroll, not just scribing one into your spellbook.

 
Thanks for the feedback :) To be honest, I really really don't like the time requirement for spell scrolls. If you're gonna tell your players "it's gonna take 2 months to craft your spell scroll" but never give them the opportunity to realistically take the downtime for it, you might as well just tell them "no, I won't let you do it", that's more honest. I do, however, understand the need for long periods of downtime to craft magic items, because otherwise, your players might start hoarding them. But that makes no sense in the case of creating spells: It's not a resource that you can accumulate.

I'll take a look at the book you mentioned, though. Maybe It will give me some ideas :)

I'm using the same method as I'm using for my tool proficiency training homebrew rules. In short, there's a cumulative DC (for a tool or instrument, it's 300). You can practice for a proficiency either with a teacher or without and you can do little bit every day or you can spend a full work day. Every day the character throws a d20. On a 1, no progress is made and a small complication, on a nat20 they get a bonus. If they spend a little while ("passive") they get a static increase to their cumulative. If they use a work day ("active") they get whatever they rolled + a suitable modifier (e.g. DEX when practicing tinkerer's tools). If they practice without a teacher, their progress is halved. There are quite a few self-taught experts in real life, so I don't think it's mandatory to have a teacher, but obviously it'll take longer if you're figuring things out on your own.

So, for a spell, they're allowed to do the same, but there's probably no teacher involved. I've reasoned this as it's the nature of wizards to learn their magic through intensive training and exercise, so they're naturally curious in trying out new arcane ideas. The DC varies on spell level and obviously no one can create spells they don't have spell slot levels for. Even a level 1 wizard can create new spells, but they're only either cantrips or level 1 spells. This cumulative DC process also gives me a little time to think about the spell and balancing before they actually manage to create the spell. So far my party (of level 4 characters) have only thought of one custom spell they'd like, which is to detect if an item is cursed or not without attuning to it. I allowed the wizard to start practicing it, but in order to master it, they'd actually need to be in close proximity to some cursed items in order to do their experiments.

Guys, just let the players talk with the DM. They have to tell the DM what spell school they want, and what they want the spel to do.

You can homebrew it then.

Yes, players have been doing this for as long as the game has existed.  Just figure out what you want and compare it to other spells of the same level.  Too good?  Move it up a level or scale down the effects.