This would be a desktop-only enhancement and would be good to pair with #4102.

You'd click the inserter and get the interface you're used to:

When hovering a block, a little preview area would pop up below the inserter:

The thing we'd show here would be the block, pre-filled with some dummy content (in this case the description text). The entire preview element would then be scaled down to 75%, using something like transform: scale( .75 ).

This was an idea @mtias suggested a while back.