Its unique feature will be a sharp jab

First off, here is a list of OSRS Items everything not released yet, in terms of melee: Warhammer, Short Sword, Hasta, Kiteshield. Wow! That was an even smaller list than I imagined.

The war-hammer could be released alongside a brand new Dwarf quest or area, or perhaps Fremmenik. Its special attack would be something useful for PvP scenarios like Clan Wars or Bounty worlds. My reason for that is that plate armor is less resistant to crush attacks, and the war-hammer would be the ultimate crush weapon. It could be a less powerful, non-degrading version of the Statius' warhammer which could be used in PvM situations as well.

Short Sword. The Obsidian and leaf-bladed swords are both strong contenders to this weapon. It's likely to appear like the Leaf-bladed Sword, however, it would have an entirely different weapon. I'm not sure of the details of its special attack, however.

Hasta. This would be released along with a Barbarian/Fremennik-related quest. Its unique feature will be a sharp jab. It will cause double damage and use 50 or more of the specially designed bars. It could penetrate small targets and hit the entire area behind it. For bigger targets, the second hit would cause double damage. The second hit would always cause only half the damage to the target.

Kiteshield. I'm not sure if it serves a purpose, as we already have the DFS as well as the other Spirit shields. Perhaps just a sturdy non-degradable shield that is worn as a sign of Buy RuneScape Gold prosperity?