Previously, I hated the constant “Remove Hunter’s Mark” posts, or rolled my eyes at the sheer number of people all pointing out the obvious issue of your core feature just being a 1st-level spell, but I’ve finally switched over to the extreme that the Hunter’s Mark spell just shouldn’t exist at all.
My only defense for keeping it was that it has the clear thematic identity of “Choose your prey, and hunt them down over time,” which I liked for Rangers, (Especially over “Yeah, you’re either mediocre under hyper-specific circumstances, or literally invalidate the rules of the very encounters you’re supposed to be good at.”) but something about it still left me unsatisfied. I’ve finally realized the solution that should have been obvious: Focus on that identity for a feature, but make it something better than “Could I offer you a d6 in these trying times?”
Favored Foe, or Favored Enemy, or even Hunter’s Mark if you really like the name, should take the concept of select a target and apply a personal debuff for a duration, but make it give something other than strictly damage. Have a bonus to hit, or some utility options, or whatever. Make the Ranger’s true damage come from some of the amazing spells they have, but updated to be more usable, like the new Paladin’s Smite feature.
Tbf, Feral Senses is also pretty much an optional Warlock feature, arguably Warlock’s one is better and at a lower level. Truesight is better than Blindsight unless you us the Fog Cloud spell or get Blinded often.
Half-caster Warlock is a scale model of the martial-caster divide. They could put almost any level 18 or below feature as an Invocation and people would probably say it’s worse than Mystic Arcanum at that level.