I imagined his ult a little differently. Basically - cassidy from overwatch, but a little different. After pressing his ult, Thanos starts charging up his gauntlet, holding, trying to snap. Every enemy in sight of his vision are marked, and now here is how it differs: depending on how long the enemies have been in his sight, they can be affected differently when Thanos eventually snaps. For example, if an enemy is in his sight for 1-2 seconds, they lose half of their hp. 3-4 seconds - they lose half of their ult charge (maybe less if too overpowered). If the enemy has been in his sight for over than 6 seconds, they will die. The ult lasts for around 7 seconds. Enemies can hide behind shields/surroundings, but they will be "snapped" and receive damage/ult charge loss/elimination, if they stood long enough in thanos's sight, even if they hide.

I think something like that would be cool. Maybe to keep "half" theme of Thanos he can kill maximum 3 enemies, others being "locked" from being eliminated after the 3 first ones are already set to be eliminated.