This should help both single and multi player, including Crucible.
Split health bar into 2, for example, upper half showing your actual health (same as now), while the second half showing your estimated health after all DOTs (damage over time effects) are done. Can be shaded, light red and dark red, for visual appeal. Or just draw a marker on where you health is “headed”, on top of the health bar.
This is useful to determine when to get out of fight. Without this feature and while playing a caster build, it’s often too late. Especially useful on ultimate, where DOTs often account for 90% of the damage you take, and a typical DOT eats at least half of your health (7K in my case). So 3-4 of those and potions can’t save you.