Make them a charge oriented unit again. Like d3 attacks with Bladevanes on the charge, maybe throw in some rending, and make the Grav Talon and Calatrops more reliable/actually do something other than cause 1 mortal wound sometimes maybe half of the time.
Let's break this problem down with some math. In their current iteration, Reavers lack the offensive oomph to compete with our other melee units.
First, let's find out the offensive output of your average Reaver.
They get two shots with a Splinter Rifle.
2 Attacks -> 1.33 Hits -> .67 Wounds -> .44 Dead Guardsmen -> .22 Dead Marines.
In Assault, you get,
2 Attacks -> 1.33 Hits -> .89 Wounds on T3 -> .67 Wounds on T4 -> .74 Dead Guardsmen -> .33 Dead Marines.
For a grand total of 1.18 Dead Guardsmen and .55 Dead Marines.
Already seems pretty lackluster, but let's compare it to equivalent points in some other assault units.
Starting off with three Wyches, firing their Splinter Pistols.
3 Attacks -> 2 Hits -> 1 Wound -> .67 Dead Guardsmen -> .33 Dead Marines.
In assault, they're granted,
6 Attacks -> 4 Hits -> 2 Wounds on T3 -> 1.33 Wounds on T4 -> 1.33 Dead Guardsmen -> .44 Dead Marines.
For a total of, 2 Dead Guardsmen and .77 Dead Marines.
Already I think we can see the problem. A unit in the Troops Slot, with one more Wound, a 4++ Invuln and the ability to tarpit already outshines the bikes...while costing 3 Points less overall.
Let's move on to Hellions.
First with their Splinter Pods
6 Attacks -> 4 Hits -> 2 Wounds -> 1.33 Dead Guardsmen -> .67 Dead Marines.
Next in Assault
6 Attacks-> 4 Hits -> 2.67 Wounds against T3 -> 2 Wounds against T4 -> 1.78 Dead Guardsmen -> .67 Dead Marines.
For a total of, 3.11 Dead Guardsmen, 1.33 Dead Marines.
Now while this is four points more expensive, that hardly excuses a more than 100% increase in offensive power, and while they certainly suffer from a lower survivability, their Hit and Run ability allows them to be a versatile annoyance to the enemy.
Finally, let's take a look at Khymeras, no shooting, and we'll assume there's no Beastmaster nearby.
9 Attacks -> 6 Hits -> 4 Wounds against T3 -> 3 Wounds against T4 -> 2.67 Dead Guardsmen -> 1 Dead Marine.
No need for a Grand total here. While they aren't exactly maneuverable, even Khymeras leave Reavers in the dust...while having an Invuln and equal Toughness to boot.
Thus highlights the problem with Reavers, there's not much that they can do that other units can't fulfil better.
Now let's look at my proposal, d3 Bladevane attacks when having charged with Rending. (6+ to Wound resolved at AP -3)
4 Attacks -> 2.66 Hits -> 1.33 Normal Wounds on T3 -> .89 Normal Wounds on T4 -> .44 Rending Wounds -> 1.55 Dead Guardsmen -> .81 Dead Marines.
Adding to their previous shooting gets us..2 Dead Guardsmen and 1.03 Dead Marines.
Still not the best of the lot, but that, combined with its decent movement and survivability, would at least make it competitive when compared to the rest of the index.
"Death solves all problems. No man, no problem."