Doborog probably won't look at this.
First, we must define our 2 main problems.

1: Weapon needs to be unique.
Each of the 4 main weapons have their functions that make them special. Sword is the all rounder, Bow is the close/ranged combat weapon, Hammer is the unblockable destroyer, that blocks nothing, and spear is a sock on a stick with a peace of cardboard.
This isn't a big problem, we just can't repeat the above functions, or make the new weapon too overpowered.

2: Upgrade Tree.
The upgrade tree is too large. even if one weapon were to be added, some existing upgrades may begin to overlap, as seen in the Unlock Hidden Upgrades mod. I have 2 solutions to stop this problem: you could make the upgrade screen an overhaul. by using the movement controls, you could glide across the screen, and with the mousewheel/D-pad, (on controller), you could zoom in and out. Some upgrade icons will be hidden after zooming out enough to stop them from overlapping. Next, you can take inspiration from Modbot, by separating each upgrade branch into it's own section. the Main section would house Kick, Energy, Jetpack, Repair, Armor, Clones, and Team Revives. The main weapons will have their own section.

Now let's talk about the new weapon.

Crossbow (Shockbolt Crossbow)
Basic Description: Designed to give the user an opening for a killing shot.
The Crossbow is the only weapon that can store a projectile, unlike the bow, you can hold multiple projectiles at once, and even unequip the weapon to shoot the projectile(s) later.
These projectiles will add an unused function to the game: the E.M.P. effect! This would be updated, however, to nerf the effect. First, it will now just stun the player, not completely destroy their armor, and the energy upgrade branch strikes again! New upgrades will be added, the first being Stun Reduction. This is Get Up, but specifically for the E.M.P. effect.
This upgrade also has another function. The E.M.P. effect now slowly takes away energy for it's duration, (about the same amount of energy it takes to jump in L.B.S.). The Stun Reduction decreases how much energy is taken by half. there will be 2 Stun Reduction upgrades. You must get kick with the Crossbow. Why? Because of a specific function it has. If you loose your non' dominant arm, and the Crossbow still has munition, you can still use it, you just can't reload it. once there are no more bolts in the Crossbow, you drop the weapon. Now let's talk about upgrades! The Crossbow will have 2 upgrades that branch off of it. The first is Bolt Capacity, (which, as the name suggests, increases how many projectiles the Crossbow can hold). Next would be Charge Reduction, (which decreases the amount of time it takes to load the crossbow). Now we talk about the new projectile: The Bolt! The Bolt is the Crossbow's projectile. It travels 3x the speed of an arrow. with this power, it easily shoots through shields, and applies the Unbalancing 2 effect on the sword, (if blocked), however, the bolt would be absorbed by the sword, rather than being deflected, giving the sword the E.M.P. effect for 15-30 seconds. The Bold can still do damage, it's just so small, it can't really kill anything that it hits, unless the target was stunned. The crossbow is more likely to kill it's target after two shots, which is why at it's base upgrade level, it holds up to two Bolts.

(More weapons are to be suggested)