~Cho'Gaths indicator for death really means that if you use your execution at that exact moment the enemy champion would die. So yes he could use it, he's supposed to be an easier champion.
In regards of Zed, his indicator is only shown after he's applied enough damage on his Death Mark. This is both a good and bad thing for the Zed.
~The good thing for Zed is that he knows he applied enough damage to kill, so he could try to leave and let the Death Mark finish the enemy off. However, it could give him a false sense of security and the allied saved by someone else before the Death Mark goes off, It's not something like Cho'Gaths that guarantee's a kill if used properly.
~Garen does have a execution indicator but only on his "Villain". I'm totally fine with this, his extremely simplistic kit works very well this way, adding a indicator regularly could possibly make him just slightly too easy, The problem with this is that Garen is supposed to use his ult when he thinks it will execute, If he's given an indicator, his ult could never possibly miss or be used too early, leaving little room to counter play unless you use a heal/shield right before he uses his ult.
~When it comes to Darius, I think a indicator for his ult could be too strong because once his axe comes down, you have very little time to react to it, ontop of it there is already a 5 blood stack indicator with the addition of his passive granting him 5 blood stacks on enemies after the first, This could lead to instant dunks on multiple targets that would normally leave Darius wondering if it would kill, A resettable execution in my opinion should not have an Indicator.
~Veigars ultimate technically isn't an execution, it's just a huge damage ability, I've had times where it was late game, and it would ONLY take the press of R to kill the enemy ADC or Midlaner, Then it would show a permanent indicator for them, this would remove forms of counter-play besides (Don't get close to him) because it leaves out the calculation of the base damage + your AP, + 80% of the enemy AP that you'd normally need to do on your own. So keep in mind, His ultimate is meant to be a high damage ability, not execution, his execution is his Q.
IM NOT SHUTTING DOWN YOUR IDEA, I LIKE IT, BUT NOT FOR THESE CHAMPIONS IN SPECIFIC
and 
could have an indicator, why not? I believe it would add a sense of timing for those who are sick of seeing that one 
running away with 10 hp and spamming laugh.
- A circle of spinning swords around the enemy, sort of like Garen's W animation.
- This one is a bit odd to think up due to his passive already having an indicator of "Oh god I'm dead", but I think it would make for sense for the Noxian Might indicator on the opponent to be smaller, but have an extra emblem on it if the enemy can be dunked. Like two axe-heads on the sides? For Dunkmaster, I have no clue, any opinions?
- An icon above the enemy's head showing Veigar's glowing yellow eyes, something simple. For Final Boss Veigar, an icon stating "GAME OVER".
