Having a more advanced heavy attack would be cool.

[Colies]

I'm not saying that the heavy attack we have now needs any change, What I mean is just simply adding another attacking move.

Hold the heavy attack button for a few seconds  (about 3 to 5 times the wind-up time) to use it, very simple.
And...yes, holding the normal attack button would only use the basic heavy attack.
Release the button earlier will also still use the basic heavy attack.

Here is a list of attack moves you may take as a reference, Dear DE.

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Assault Saw

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Throw the saw forward, jump kick at the saw, the kick will cause a mid-range shock wave, then pick up the saw and do a 360 chop.

Blade and Whip / Whip

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Do corkscrew trick jump rotate rapidly, that strike enemies in front and behind, end with a ground slam.

Claws
 

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Basically is the ground finisher but forward, strikes much more times, and is swifter.

Dagger

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Reuse the current heavy attack but more swiftly (like almost instantly), adding a few afterimage effects to strike more times.
Fast and clean.

Dual Daggers / Dual Swords

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it's a little complicated, you know Monsoon?
well, have a look at this video, it's easier to understand.
https://youtu.be/TBH_Yo5sSEI?t=820
No need to make warframe turn into pieces like Monsoon though.

Fist

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Slam the ground slowly but forceful, hit a much larger area, come with a long-range shock wave.

Glaive

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Just bounce a few more times, but force it to redirect to the enemy, which means each bounce is guaranteed to hit the enemy in range.
Automatically explode on the first hit.

Gunblade

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Shoot laser, yes.

Hammer

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Instantly teleport forward for a short-range (or to a nearby enemy if you guys are willing to do a little more code work) and do a heavy slam(Reuse the current heavy attack move will do) that cause a long-range shock wave.

Heavy Blade

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Do a top-down ground slam that causes a mid-range shock wave, then jump into the air with a front flip and throw the weapon forward, rapidly slide toward the weapon and pick the weapon do a slide attack backward that causes a short-range wide scope sword beam.

Heavy Scythe / Scythe

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Just reuse the current heavy attack but more swiftly, Strike twice, each strike causing a mid-rang 180-degree shock wave.
Heavy Scythe will cause a mid-range sword beam as an extra bonus.

Machete

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Swing it like using nunchucks swiftly, and end with a ground slam that comes with a mid-range shock wave. 

Nikana

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Rapidly slashing many times while dashing forward for a short distance.
Each slashing cause a short-range sword beam.

Nunchaku

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Peel the ground forward to make a shock wave and the ground thing forward to hit the enemy in front, middle range, then end with a full extended side slam attack.

Polearm

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Kick the enemies in front, suspends the enemies in the air, then do a 360 spin attack that causes a short-range 360 sword beam.

Rapier

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Swing it vigorously to cause a mid-range shock wave, then stab forward to cause a long-range shock wave forward (but with less scope).

Sparring

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Fly kick forward, Kick slam the ground as a counterforce jump back to position, and also cause a ground slam while back in position.
The 2 slams come with mid-range shock waves.

Staff

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Kick the enemies in front, suspends the enemies in the air, then jump to the enemies and slam the ground causing a mid-range shock wave.

Sword

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Rapid top-down cut with a squat pose(Or reuse the current heavy attack move), remain the pose for a bit, and after half second, a strong wind comes and strikes all the mid-range enemies.

Sword and Shield

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I have no idea, haven't touched shield stuff so much.
Maybe a fast shield charge?

Tonfa

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Rotate the Tonfa on both sides rapidly for 1 second, pull left and right side enemies toward you, and hit the tips on both sides of the Tonfa against each other, causing a mid-range explosion.
If the weapon doesn't have any element damage then causes strong wind instead of explosion.

Two-Handed Nikana

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Reuse the current heavy attack but more swiftly that shoot a slow, long-range wide scope sword beam.

Warfan 

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Spinning in place for a few seconds, constantly blowing strong wind to tear all enemies near you.
Maintaining the movement will make the wind larger, cancel the movement will stop the wind immediately.
yeah, recycling the old fashion Telos Boltace, haha.

 

This doesn't affect the balance between gun and melee in the high-end game I believe, just for having more fun in the game, after all, it required you to stay still for a few seconds.

Just another decorating thing, just like the wall attack but way cooler.