One idea, read in a second
A thumbnail competes at roughly the size of a postage stamp, next to five other videos, on a screen someone is scrolling past. Anything that needs a second look is already lost. One subject, one contrast, one readable word or none at all.
Design it next to the title
The thumbnail and the title are one unit. If both say the same thing, half the space is wasted. If they contradict each other, the click feels like a trick. Written together, they can set up a question and leave the answer inside the video.
If it does not read at phone size, it does not exist.
Test the small version
- Look at it at 20% before anything else
- Remove one element, then check if anything was actually lost
- Compare it against the videos it will sit beside, not against itself