Some ideas to improve single-target melee skills

Glacial Hammer: Killing a frozen enemy causes a cold explosion - similar to Herald of Ice, but scaling with monster life times how long the target had left on its longest freeze timer at the point of death (including the freeze added by the killing blow if applicable).

Heavy Strike: Remove the knockback. Applies a stacking debuff that makes the enemy easier and easier to stun.

Double Strike: Change to Multiple Strike and make it a channelled skill that repeatedly hits enemies in melee range at a high rate (but each hit gets a separate crit roll etc). Multistrike-style targeting. Stun immunity while channelling.

Puncture: In addition to causing bleed, applies a special debuff that causes the target to take 'moving' bleed damage even while stationary.

Elemental Hit: When used with a melee weapon, guaranteed freeze/shock/ignite depending on which element is rolled. So e.g. if you roll added fire damage and your weapon already does lightning damage, you'll get a guaranteed ignite, but shock will be determined based on the normal rules. Alternatively: hits ignore enemy resistances.

Viper Strike: Poison inflicted by this skill spreads on death (same effect as on Bino's Kitchen Knife, doesn't stack with it).

Dual Strike: Can't be evaded.

Vigilant Strike: maybe reduce damage slightly, but remove the cooldown. (There's not much point in this skill as an occasional attack, because it's already easy to get Fortify by other means.)