The basic idea of this weapon in mind would be similar to a wizard's staff in fantasy movies/stories, only that it uses the player's own energy in the form of damage. This idea can be executed in many ways, but I've gone and formed a idea based around this weapon myself, so I would appreciate feedback on this.
So this staff in mind will be unlock in mid-late game and has two attacks, a ranged energy spear/arrow to pierce through your enemies, and a long energy sword to slice your opponents. However, the staff itself needs to charge before it can attack, charging for 1 second will use one bar of energy, 2 seconds will use two bars, 3 seconds will use 3 bars, and 4 seconds will use 4 bars, I will rank these in tiers for better reference. The power of the staff's attack are proportional to the energy used.
A tier 1 energy sword attack from the staff will only be around the same length and width of the default sword you start with, not including the length of the staff itself. The length and width of the sword will dramatically increase proportional to its tier, the sword itself cannot be block by any means, and can go right through your enemies. The speed of the sword, no matter the tier, will be closer to that as the upcoming hammer.
The energy spear attack can be similar to the bow, a tier 1 energy spear is only around the same width as a mk2 robot's arrow, only that it has a limited range, and travels at a lower velocity. Again, like the energy sword, the spear's size, and range will increase proportional to its tier, but the velocity stays the same so its easier to avoid. The spear is completely unblock-able and can go straight through your enemies.
Although this weapon may seem to be incredibly powerful, even though it is, there are still some drawbacks that come with it. During the charging phase, the player will expect a heavy movement speed penalty, perhaps by 30% or so. This will make the player extremely vulnerable. And since the player would not be able to block both swords or arrows, he will be an easy target for archers, swordbots, or spidertron. If the player were to cancel the charge or be knocked back, the charge will be interrupted, although half the energy used will be recovered. There is still a punishment, but the player will at least have the chance to retreat. So timing and positioning will be vital when using this weapon. On the other hand, the player can jetpack whiles charging without a speed penalty, but due to two separate actions sharing energy, the player will not be able to get a very powerful attack nor go very far with the jetpack. Also, the speed penalty may not apply or be significantly reduced when the player loses a leg.
Now, I don't really think their should too much upgrades available for this weapon, considering the player will need to spend a lot points on energy capacity just to get the weapon it it's full potential. Nevertheless, there should be upgrades to at least reduce the charge time and speed penalty.
With all things considered, this idea of a weapon is based around a risk/reward factor, the player is putting himself in great danger just by charging the weapon, but a successful attack, especially one that outright kills 5 to 10 enemies, and possibly completing an entire level, will be very rewarding and satisfying. Even through its basically a combination of the default sword and bow, it's much more expensive to use and is not recommended to spam attacks and only kill one enemy at a time. To get the best bang for your buck, the player should try to go for the most kills as possible for a single attack, encouraging grouping the enemies and putting yourself at greater risk of getting surrounded rather then separating them.
So, if you have any thoughts of this weapon you would like to share, perhaps a better way to balance it, I will be grateful. Also, I would like for you to share any different ideas you may have based around a staff-like weapon. So thank you for taking the time to read through my massive wall of text.