[RPG] How to make new playable races

The traits of the aquatic elf (Monster Manual 103) are listed as extraordinary abilities in the Monster Manual (2003) and in the SRD, but they are not listed as extraordinary abilities by the premium edition Monster Manual (2012), therefore they're actually natural abilities via stealth errata. (This change is consistent throughout the premium edition and, for example, eliminates some confusion with races that had all their racial traits listed as extraordinary abilities even when those extraordinary abilities were also spell-like abilities, like some possessed by a drow (103) or a gnome (131).)

With this in mind, this DM would consider the natural ability gills of the aquatic elf a gross physical quality of the creature, therefore acquired by a creature when it assumes aquatic elf form using the 2nd-level Sor/Wiz spell alter self [trans] (Player's Handbook 197).

(It's fortunate for humanoid casters who want to breathe underwater that the aquatic elf specifically relies on the racial trait gills to breathe underwater. Many sea creatures—even humanoids like the locathah (MM 169-70)—instead rely on the subtype aquatic to breathe underwater, yet, while the subtype aquatic is specifically granted by, for example, the supernatural ability alternate form (and effects based on it like wild shape), the spell alter self doesn't add or change a creature's subtypes; getting types and subtypes takes an effect like the 4th-level Sor/Wiz spell polymorph [trans] (PH 263).)

I spent about an hour looking, but I can no longer find the article, interview, video, or tweet I'm looking for. However, I'm certain that at some point there was a discussion with the designers about how much a given PC race's vision type counted for balance, and the response was basically, "none at all." In other words, the game designers did not rank PC darkvision as being significantly more valuable than normal vision for balance purposes, and treated it as a flavor component of each race.

I know a lot of players disagree, probably because every player seems to have a self-sufficiency reflex. And the Darkvision spell seems anomalous until you realize that it's 2nd level because it's specifically for when the narrow uses of darkvision exactly come into play. It's the same reason Knock and Spider Climb are 2nd level. They're not needed all that often, but when they are they're very useful.

Keep in mind that if you're following the game's rules that complete darkness is dim light to a character with darkvision, then characters with darkvision wandering around without a light source is only slightly less stupid than it is for characters with normal vision (-5 to passive Perception and disadvantage on Perception rolls). Such a party is all but guaranteed to trip every trap in the dungeon, and tasks like reading are likely going to be flat out impossible unless the print is particularly large. Even a party of Dwarves should be using a light source in an unfamiliar tunnel. Though light sources can be seen by other creatures, sound is often a bigger giveaway about the presence of other creatures.

Bottom line is that, from a design perspective, it's apparently not worth very much. At that level, I would grant an additional language, vehicle proficiency, or tool proficiency. Alternately, you might grant perception advantage in certain circumstances (e.g., Dwarves and Rock Gnomes underground, Elves and Forest Gnomes in the wilderness, etc.).