One of the things I've noticed that makes the Psyker so incredibly powerful and to a much lesser extent the Tech Adept, is their ability to individually customize Psyker Powers and Constructs.

In order to bring Crusaders and Assassins more in line with this I suggest an Armory Bench where weapons can be customized.

Add 3 modification slots that add bonuses across all of the weapon's attacks.  

Examples:

• Sanctified Oil (ammo ranged weapons) - All attacks have X% improved cool down.
• Adamantite Structure (melee) - Increase damage of all attacks by X%
• Power Field (melee) - All attacks gain Armor Piercing
• Advanced Heat Sinks (heat ranged weapons) - Reduced heat generated by all attacks by X%.
• Shock Rounds/Shock Studs (ranged/melee) - Hits cause shock.
• Etc.

The modifications would apply to all 4 attacks since you are modifying the entire weapon to make it better and suit the individual players style.  

These could be New Skill Tree unlocks much like the Psykers Psychic Debuff and Psychic Combat trees... OR more like the Tech Adept's construct modification system and weapon mods could be drops.

Plus, if you wanted to be EXTREMELY generous, increase the pool of attacks that can be performed with a weapon.  

Currently Melee/ranged weapons have a static 4 attacks.  Increase that pool to say 6 attacks and let us pick which 4 we want to use on that weapon at the Armory Bench.