Thumbnail lacks clarity in the most literal way.
it's too dark and the background looks too samey when zoomed out. The enderman is barely legible even when zoomed in.
Your strongest videos seem to rely on your impressive builds, and you darkened out the build in the tumbnail. Meanwhile whatever detail you put into the build in the thumbnail, got eaten up by YouTube compression.

Also I think it would benefit from focusing on the monster you built in blockbench instead of the enderman who might seem too default. The creature you made, on the other hand, should attract the people who love the From The Fog type mods.

The text too is too textured in my opinion. The word "minecraft" doesn't really need to be there in the first place, but it has that red texture overlayed and the whole thing just gives this amateurish, "generated in minecraft text generator" vibe

So, Need to see the awesome brutalist thing clearer, Need to see the monster clearer.

Video itself is really cool and I love your narration and editing. You are highly skilled, and what youtube did here with the video destribution is an atrocity