Last year, the community decided on what new mob Mojang would implement into the Caves and Cliffs update. They ultimately landed on the Glow Squid, a brighter, friendlier variant of the existing normal one. Now, that mob, along with the Glow Lichen, is on display in the first-ever Caves and Cliffs snapshot and beta for Minecraft Java and Bedrock respectively.
They don't yet spawn on their own, but you can drop them into the world with a spawn egg. They illuminate the surrounding area, acting as a light source, which could make for some interesting underwater base designs.
What do they drop? Well, glow ink sacs which, when applied to signs, make the text hyper-visible so that you can read them at night. The normal ink sacs reverse this effect. If you craft an item frame with this glow ink sac, any items inside will also be lit up. However, there is a hunter to the poor Glow Squid as Axolotls will attack them on sight.
Dripstone blocks have also been added, along with stalagmite which damages you when you land on it. Then there's the Sculk Sensor which detects steps, swimming, block placing, destroying, Elytra free falling, fall damage, splashing, and projectiles.
The full update is set to release Summer 2021, and there's still no word on whether ray-tracing will be coming to next-gen consoles, but, hopefully, Mojang will implement it alongside the Caves and Cliffs update.
As for the Glow Lichen, it's a "dim light source that generates in caves" which you can pick up with shears and use bone meal on to spread across blocks. Mojang is hell-bent on adding as many new light sources as they can, but the community is more than happy to have new ways to keep their builds mob-free.