Geoglyphs.

What are they?

These would be large-scale indicators on the surface which appear above strongholds. There could be multiple different Geoglyph designs too, so players who look from above will see a beautiful pattern.

How would they generate?

Wherever there is a stronghold, a geoglyph will appear directly above. The game would generate grass blocks on the surface which generate in patterns. These patterns could be generated using perlin noise. 

The grass blocks would have a bright colouration to them, so they appear almost glowy. 

When a stronghold generates below an ocean, the geoglyph pattern would translate to water, so the water colour would appear lighter and more vibrant and form these perlin noise patterns.

Why would they generate?

Strongholds are uninteresting to come across, especially when you appear at the location on the surface. Having a large marker would be quite interesting and could lean into the storytelling aspect of Minecraft.

These geoglyphs could also provide hints to things in the game, such as sniffers, withers and the warden.

Geoglyphs could also lean into lore, and could provide players an interesting area to build on the surface and could inspire storytelling. From the ground, they would look like glowy patterns, but from an aerial view, players would see the full picture, inspiring players to build towers or fly over with elytra to see what geolgyph has formed.

I think this feature would fight right at home in minecraft, inspiring stories and wonder.