This is a theoretical situation, but I wanted public opinion on the ruling.
Heat Metal states (ignoring the "At Higher Levels" part):
Choose a manufactured metal object, such as a metal weapon or a suit of heavy or medium metal armor, that you can see within range. You cause the object to glow red-hot. Any creature in physical contact with the object takes 2d8 fire damage when you cast the spell. Until the spell ends, you can use a bonus action on each of your subsequent turns to cause this damage again.
If a creature is holding or wearing the object and takes the damage from it, the creature must succeed on a Constitution saving throw or drop the object if it can. If it doesn't drop the object, it has disadvantage on attack rolls and ability checks until the start of your next turn.
So, strictly RAW, Benny the bard casts Heat Metal on Freddie the fighter's sword. Freddie drops it, either due to failing the save, or on his turn as a choice, or due to any other reason. First of all, it seems the spell would continue if concentration was maintained, because the spell targeted the sword, not Freddie. So if someone picked it up and held it, Benny could repeat the damage on the bearer (even a new bearer) with his next bonus action.
Suppose, though, that on Freddie's turn, he picks up his sword, attacks with it any number of times, then drops it (which doesn't require any action). Benny's turn comes and no one is holding the sword, so he can use his bonus action for anything else, and chooses to maintain concentration. On Freddie's turn, he repeats this, picking up the sword, using it (with no disadvantage, since he didn't take the damage on Benny's last turn, regardless of how the first save went), then drops it. Repeat for the full minute.
Even if Benny was choosing to use his bonus action on the spell every turn (and I think he could, because he's flaring up the object, and it's not the object's fault no one is in physical contact with it), I don't believe that Freddie would take any damage, nor would he suffer disadvantage, since he's not holding it on Benny's turn when the bonus action effect is triggered. And yet he'd get to use the sword normally each turn, subject to the normal limitations of action economy.
Yes, someone should probably move the sword. Yes, Freddie can't use it for opportunity attacks or any of his other off-turn tricks or abilities. Yes, yes, yes, all sorts of contingencies and ways this could be interrupted and issues with continuing the fight this way. I'm not looking for a viable strategy, I just want to see if there's anything missing from my reading of the spell that would make this gravity-boomerang method result in no further damage.