It's also easier if the currency is valuable IME, this works better in D&D 3.5 and Pathfinder than in D&D 5e simply because wealth in 5e is (IME) less relevant - there's far less to spend GP on in 5e after about 3rd level (when you've got the weapon and armor you want) except for costly material components.ДЄssuming that the assumptions can be assumed: auction the thing off to your players. This is much easier with stuff that can be purchased individually rather (eg., with gold or GURPS-style XP) than stuff that comes with D&D-style levels. So, let the theory-crafting begin! AuctionДЄ quick option to ballpark the value of a discrete homebrewed thing is to auction it off to your players. And, it's important to remember that 'problematic' doesn't necessarily mean 'overpowered' - an underwhelming or underpowered option can be just as bad from the player's perspective (though it may be easier to resolve, since the Endowment Effect is less likely to be an issue). However, there are a few bits of theory-crafting that can help identify homebrew stuff that may be problematic. Either way, however, the question is still relevant for some of the other features I'm Markov's answer is definitely right: playtesting - and playtesting a lot, with a lot of different groups in a lot of different scenarios with a lot of different party compositions - is the only way to be sure. Since I was thinking this would be overpowered at level 1, it's possible I've been thinking too restrictively. The only difference is that the replacement spell has to be of the same level as the spell it replaces, not the same or lower level. But in the meantime, what principles can I use (in addition to the ones in the Unearthed Arcana article) to determine whether and at what level a new optional class feature is balanced?Д®DIT: Since posting this, I've found out that the UA playtest class features give almost exactly my example as a feature for the Bard, Ranger, Sorcerer, and Warlock classes at 1st level. I've only been playing 5e for a few months so maybe when I have more experience it'll be easier. I've looked at other classes and can't find anything that really 'feels' the same, so I'm having trouble using that as a guide. But maybe that's totally bonkers and it would be perfect at 1st level, or maybe it would break the game. I feel like this might be inappropriate at early levels, when the Sorcerer knows so few spells, but might be more appropriate later on, when switching out one spell would leave the character's spells known mostly unaffected.

When you finish a long rest, you can choose one of the sorcerer spells you know and replace it with another spell from the sorcerer spell list of the same or lower level.

This feels a little too restrictive to me, and I'd like to add the following benefit: How do I figure out: (1) whether they're overpowered/underpowered, and, if they're not, (2) what level they'd be appropriate at?ДЇor example, the sorcerer's Spellcasting class feature currently includes the following text:ДЄdditionally, when you gain a level in this class, you can choose one of the sorcerer spells you know and replace it with another spell from the sorcerer spell list, which also must be of a level for which you have spell slots. Obviously the safest solution is to rule them out, but I'd like to try to be more thoughtful than that. I can't tell, though, whether a few features I'm considering adhere to or violate the principles in the Unearthed Arcana article. the sorcerer can use her body as an Arcane Focus from the get-go). Some of the features I'm adding seem so narratively essential to the class and also so mechanically negligible, at least in the campaigns I've played in so far, that it's easy to figure out where to put them (e.g. I've read the Unearthed Arcana article about modifying classes, and I've tried to adhere to all the principles it sets forth. I'm working on a homebrew revision (and slight buff) of the Sorcerer class in D&D 5e that includes some additional class features.

Homebrew d&d classes