Like, does anyone actually fear this trait? I was thinking about changing it to something more likely to kill you

I think the only trait I ever actually take note of and make special effort to kill is crippling aura. The rest are harmful, but not the end of the world. But yes, sprinter is pretty useless, I can count on one hand the number of times I've had a mob actually get away and bring back an add and it certainly never happens late game.

Was thinking about changing it to something like Leader. He'd summon reinforcements at 50% health (pulls one mob). Or, better yet, he'd summon the reinforcements after a fixed amount of time ... like 30 seconds. But if you kill him first an add doesn't get pulled. I think this would be awesome because it would add strategy to what order you target mobs. Obviously a Leader would be a high priority.

Reasonable, could just be a forced ambush. Only restriction I'd suggest is the mob he pulls can't be a rare / champ / boss. Otherwise you might see "leader chains" which are going to be RNG certain deaths to characters not prepared for chaining.

Interesting idea. I like the anticipation of impending doom. Maybe add an emote declaring that the Leader has called in reinforcements some time period before they join in.

"Soandso sounds a warhorn. Reinforcements scramble to join the fight!"

Bonus points if you can have it play a clarion call with the pitch bent to scale with the mob's size. Low warhorns for large orcs and giants and tiny high pitched trumpets for pixies, goblins, and rats.

To be perfectly honest the only champion trait I ever notice is the one that makes you swap targets every 1 second and maybe juggernaut.. the others I hardly notice because they die in seconds.

Frenzied is the nasty mofo to watch out for.

To be perfectly honest, I have yet to ever have a mob escape in either Beta or Live. (Make them run faster?)

Never happened in my playtime that a sprinter ran away successfully, turning it to Leader looks interesting.

One trait I look for is Martyr, after one of them killed me... Frenzied is nasty too ( but you can see it right away).