How the devs design a Vanguard
Take a beloved Marvel powerhouse.
Make them HUGE so they're really easy to hit.
Increase their HP. Not proportionally though. Make sure it is not enough to compensate for them being so huge.
Reduce their damage to 40% is what the character should logically be doing.
Give them an ult that is really bad on it's own, but could facilitate someone else getting MVP.
Do their attacks flow well? Add some animation delays in there and increase cooldowns until they don't.
Do they have a regular normal attack like a ranged shot that most Duelists get as their Mouse 1? Ok, put a hard range limit on it to like 25 meters, make it root the hero in place, add a telegraphed delay and make it a slow moving projectile and give it 10s cooldown.
Have you considered MAKING UP a resource they have to manage? Like, Juggernaut is literally inexhaustible, but how about he can't attack or move until he charges up enough Cytorak Ampules?
Can they move about freely? Is this helping them survive or attack? Consider longer cooldowns. Can this character fly? Well not here! Are they a melee hero? Consider making their melee attacks SLOW them so that their targets can backpedal faster than the Vanguard can advance while attacking. The goal here is go make it as frustrating for the Vanguard as possible while maintaining the illusion they can catch their target.
If in live testing it turns out nobody wants to play the hero after a week, this is a sign of "Fun Fatigue" and that clearly your new Vanguard is too powerful and needs to be made more terrible. Consider nerfing their damage, movement and health randomly for months to come. When do you stop? Who said you have to? There is no end goal, so long as they are worse than if they had been made a Duelist.