To remain lore friendly: realistically to have access to a 10th level spell: Realize that they were never bestowed, the formula for 10th level spells were never taught, they always had to be researched by a wizard who was hundreds of years old and not a lich.

So longevity spells that fully sustained the wizard were constantly in place.

The first one was always the spell that created floating cities, by cleaving the top of mountains off flipping them upside down while at the same time creating a lodestone that would sustain their flights. It took millions of gold and tens of thousands of man hours to research and then cast the spell. 

Once they established their city, they had a ready supply of income, man power and test subjects: The arch magi were not benevolent beings. Most archmagi only managed to obtain a small number of the very few 10th level spells existance. Some managed to create a few 11th level spells and using the only known 12th level spell karsus was able to temporarily borrow mystyls power..took 6 hours, a gold dragon (lawful good) gizzard stuffed with stones tarrasque blood and bile from a 12 headed hydra. So in no way was not an evil spell.

Evil and selfish: the only known spells of 10th level or higher fall into this category. The floating cities were always used to entice and subjugate. No one created a spell that restored entire battlefields to life....but they did create a spell that killed everyone in a 100 mile radius with no saving throw.... 11th level had a terraforming spell that redesigned entire sections of the planet...regardless of what the inhabitants wanted: turn an ocean into desert and force fishing villages to starve, or turn an entire mountain range into plains....

Lastly the slot in and of itself np, just don't give any spells or have a grand adventure in which the players learn why they are cut off.

Sorry for the Threadromancy, but the release of RotFM has made this a relevant discussion again. 

SPOILERS AHEAD!

Firstly, not all spells of a level have to be equally powerful. Yes, some of the 10+ spells are world breaking, but they don't have to be. RotFM has two 'Scroll' type items (NOT Spell Scrolls) found in an old Netherese city that include: Comet and Summon Tarrasque. Obviously better than 9th level spells, but not world breaking. Similarly, check out the 10th and 'Quest' level spells from the Baldur's Gate games, which included things like: Dragons Breath (more damage than meteor swarm), and Mass Raise Dead (exactly what it sounds like, also casting time was 1 action back then). Again both VERY good spells, but not game breaking. I consider Mass Raise Dead to be a bit of a gold standard; we take an effect that we know isn't broken, and just straight up multiply it, but not totally invalidate your higher level resurrection spells.

So if you want to make your own, think of some spells with good effects and then try:

-adding 50% more damage or effective damage; Comets avg damage isn't much higher than meteor swarm, but Force dmg is way better than fire and bludgeoning, and it hits a much larger area.

-More Targets: Mass Raise Dead, Mass Revivify, Mass [Good Buff Spell]

-Remove a drawback from an existing spell: Instant Raise Dead (doesn't take an hour or a diamond, or come with the usual drawbacks); Instant Simulacrum (but comes with full health), Greater Counterspell (auto counter spells of 8th or lower, 180ft range)

-Or don't make new spells but give them a 10th level slot so they can use the spells that get X bonus per higher level spell slot you use, and let them scale a spell up to 10th (12 dart magic missile! :P ) As a slightly better alternative than an additional 9th level spot.

As for "It has to better than Wish", I diagree. A lot of people look at what Wish CAN do, but not what it will LIKELY do and claim that 10th level needs to be able to do more than the greatest potential benefit from Wish, but this isn't true. It imitates an 8th level or lower spell with no drawback, but other than that you are either facing a potentially VERY steep drawback, or totally at the whims of the Gods (the DM). 10th and above just let you DO specific things. This is how they got banned. Karsus cast Karsus' Avatar (12th level) and even a Greater Goddess COULDN'T stop him, so Ao banned the spells. In theory Karsus could have cast wish and ask Mystra to steal her power, but obviously that wouldn't happen.

Ultimately, 10th level spells just have to be BETTER than 9th level, they don't have to be world ending. 

Uh, Rime of the Frost Maiden came out back when this thread was still new.

And I definitely wouldn't take inspiration from Baldur's Gate 2 because not only was that a completely different edition of the game with vastly different rules, the epic level content was made up for that game because there were no rules for playing character that high of level in 2nd Edition.