I like the 1d6 per 15' rule. Hurtling through the air toward a stone wall should do something.
In a similar vein, let's say a character was grappling an enemy next to a wall, table, etc. and decides to slam the enemies head into the object. Does that count as an improvised weapon or an un-armed attack? Seems like something that a Tavern Brawler character would love to do.