Avatar: The Last Airbender

Kinda depends what you're looking for.

• Vox Machina: More mature and 'Korra-like,' contains explicit callouts to Avatar (including a Janet Varney cameo), and is just a darkly funny and wholesome D&D adventure show. Scratches that same fantasy itch.

• DOTA: Dragon's Blood: Animated by Studio Mir in the same style as Avatar, another fantasy epic, and mature and fast-paced. The plot is kinda wild, but it still feels 'western' for lack of a better word, and I really like the characters and their VAs. There are some explicit callouts to Avatar too (like Luna's characters design and magic that looks a whole lot like bending).

• Arcane. Not a direct inspiration, but it's simply really good and "magitech"

• Pantheon. Not actually very similar to Avatar, kinda sci-fi, but watch it if you haven't. I think this is the one 'cartoon' I rate significantly above Avatar.

And some more 'mixed' recommendations I bounced off of but many fans swear by:

• Dragon Prince: The most ATLA-like, produced by an ATLA writer. As shallow as it sounds, I bounced off because of the animation style, and it felt kinda immature, but its long running and very popular among ATLA fans.

• Voltron: Legendary Defender: Sci fi, but animated by Studio Mir, and has a similar "epic fantasy" feel. I actually liked this one, but just forgot to pick it back up...

• Classic cartoons like gravity falls, she-ra, owl house, steven universe, adventure time. If you're really into it, LoK paved the way for LGBTQ relationships in many of them. I also enjoyed Young Justice and Justice League, while I'm throwing those out.