I would suggest a slight alternative to this. A buffer. Basically, let's say you have monsters on the list you have rolled for, but don't want the players to know about yet. In addition, you could have monsters leave and return to combat for whatever reason (and there are a few, but I wont get into them here). A buffer initiative would work nicely here. Basically, it sits permanently at the top of the initiative at any time it's not a player's turn, and it sits below the current turn if the next turn is not a player's. At most you would have a buffer turn as the current turn, and one underneath it to represent that the next turn is NOT a player's. Obviously, this is going to let the players know something is going on. You wont get around that, but you're not letting them know exactly what is going on. You could use it for more than just monsters (such as lair actions that you don't want to reveal the exact timing of). When it IS a player's turn, the buffer disappears entirely, only reappearing when it's no longer one of the players' turns. What this accomplishes is not showing the number or type of monsters that they can't see, while allowing them to still tell when their turn is really coming up. Maybe this is too complicated, but I hope it fosters more discussion. If anything, we could make your original suggestion a toggle on/off within the turn tracker itself.
