Tempest genasi

Certainly an interesting idea. I want to see more racial feats outside of Xanathar's Guide to Everything. And a Feat which allows Genasi to have aspects of a second element sounds awesome. The issue I take with this is that it is mechanically inconsistent.

I mean this as constructive criticism. If I were playing an Earth Genasi but I wanted the flavor of having aspects of a Fire, Water, and Air as well I could take the Magic Initiate feat and take Thunderwave as a 1st level spell and take the two cantrips Fire Bolt and Frostbite. With an already existing feat I have aspects of all 4 Genasi types in a character who may not be a primary caster. I could even focus specifically on fire and gain firebolt, control flames, and Burning Hands. That gives me essentially everything a fire genasi has except ability score increases and resistance to fire.

In the description of this feat you used the phrases "attune" and "subclass" in ways I found confusing. A race or subrace is not a subclass. And attuning is specific to some magic items. Another approach for a racial feat for Genasi which wouldn't be rendered obsolete by magic initiate is to focus on the specific advantages genasi types have, and spells higher than first level.

• Assuming this feat is taken at 4th level by a genasi, they will already have 1st level spells which they can innately cast once per short-or-long-rest. So at third level most casters gain 2nd level spells. Martial classes with some spellcasting will either gain 1st level spells here or have them from another source.
• Genasi have resistances, specific abilities like breathing under water or holding their breath, improved stealth, and innate spellcasting among other various cool aspects.
• A feat should be distinct and cool but fair. So giving access to 1st level spells and cantrips is underwhelming.
• If the Genasi is already a full spellcaster then they will have 2nd level spell slots.

Based on those 4 bullet points, I propose a fix for the feat;

Djinn Elementalist

Prerequisite: Genasi

As a mortal with ties to the elemental planes, you have deepened your understanding of the tempestuous Djinn. This has led to the emergence of new abilities. When you choose this feat regardless of your subrace you make 3 choices for resistance, 3rd level spell, and . These choices are independent of subrace and each other;

• Choose 1: You gain resistance to Lightning damage, You gain resistance to Cold damage, You gain resistance to Thunder damage, You gain resistance to Poison damage.
• Choose 1: You gain the ability to cast a 3rd level Wizard spell innately once per long rest. Choose from Fireball, Tidal Wave, Haste, or Wall of Sand.
• Choose 1: You gain a burrowing speed equal to your movement speed (increasing movement speed through spells or magic items also increase burrowing speed), You gain the ability to innately move across the surface of water or any liquid as though it were solid ground (this effect is always available but does not prevent swimming or prevent damage from walking over a hazardous liquid like boiling lava), You gain a fly speed of 30 feet (if another source would provide a fly speed, an additional 30 feet fly speed increases the total), You gain the ability to Dash or Disengage as a bonus action on each of your turns.

After this feat is taken the genasi may change one aspect of each upon leveling up.