So just a quick idea here.
I think it would be very nice if we could get a visual effect on a champion when they fall below 25% max hp when near Urgot (obviously vision of him or the target is needed) Making a bold line on their hp bar at 25%, or flashing their hp bar a few times would certainly help.
This would help Urgot players know when it is safe for them to recast their R so they don't waste it. It would also help his opponents by letting them know if shielding or healing is required so they don't waste heal/barrier. I think it is a balanced idea because the information would help both sides.
It would help curb the frustration I feel when I am in a close fight and I have to decide if they are low enough to execute or risk waiting another split second to get more damage in and possibly dying. I've just watched so many replays where I miss the execute threshold by <10 damage. I'm decently skilled at judging this after some practice, but the way hp bars are displayed makes it hard. Every level and health item an enemy builds changes the visual of their hp bar making it really hard to gauge.
Anyways, let me know what you think. Maybe do this for Garen vs his marked target as well. (or any true damage execute like Chogath's)