In Doom, it was possible to delay or prevent an enemy attack by hitting them enough that they'd flinch, interrupting any attacks or movement they were in the process of performing. With DoomRL, it cold be the case that doing over a certain amount of damage to an enemy gives a chance of them "flinching", interrupting their next turn and preventing an attack or move in your turn.
One way of balancing such a feature so that you can't infinitely stunlock enemies, and making it much harder to flinch bigger bad guys like Hell Barons could be either having the necessary damage be a specific fraction of an enemy's health, like 10-15% or more, or implement it so that you need to do above a specific threshold of damage, like with the Berserker trait, at which point a die is rolled which has to be above/ below a certain number to flinch, with a modifier applied depending on the type of enemy; i.e, you can flinch a Former Human with relative ease, but a Hell Knight will rarely do so, and the Cyberdemon is practically immune.
My other idea comes from Dungeon Crawl- when an enemy finds you in its line of sight, it calls out, alerting other nearby enemies to your presence, who then move in on you (obviously, the alerted enemies don't then call out when they see you. That would be annoying.) This would prevent players from luring enemies enemies away from groups and engaging them solo.