The basic idea is, to have extremely powerful weapons, that occupy multiple slots to balance them out.

Some of them could occupy the shield slot and one or more weapon slots, so you cant shield when they are equipped (and the shield key binding could do some special move).
Some could occupy the armor slot and one or more weapon slots, because they represent some kind of an armed suit.
Some could occupy the helmet slot and one or more weapon slots, because they represent a weapon system integrated into a special helmet.

You can only combine them, if their equipment slots don't conflict.

Here are some examples:

The Rocket Launcher would be some huge tube with a handle and occupies the shield slot and 2, 3 or 4 weapon slots. Every weapon slot represents a preset kind of rocket. So, if you have Weapon Slot Upgrades in use, you will unlock up to 2 extra rocket types. The shield key binding toggles carrying the Rocket Launcher on your back or having it ready for use on your shoulder. When it's ready for use on your shoulder your movement speed decreases. You can choose a type of rocket with the weapon switch bindings. The attack binding would fire a rocket and then have a reloading delay/animation. The charged attack would fire several rockets in a row. Types of rockets would be elemental, piercing, fire and shadow.

The Twin Blades would be two small swords and occupy the shield slot and 2 weapon slots. Every weapon slot represents a different preset attack move. One would be a fast hit that combos nicely and one would be a powerful double swing. One charge attack would similar to Cutter/Striker and the other would be an even more powerful forward swipe. The shield key binding would either be using both swords crossed to block (taking damage and breaking like a shield) or a backflip dodge.

The Heavy Battle Axe would be a huge two handed weapon and occupies the shield slot and one weapon slot. Its a powerful stun inflicting weapon and you can't shield when its equipped. You can carry additional weapons and when you switch to another weapon, the Heavy Battle Axe is worn on your back, thus protecting your back from attacks (like the Darkfang Thwacker Gremlins back is protected by a shield or like the Trojans front is invulnerable). However, it won't protect you, when you switch to it.

The Ground Shaker would be a massive bomb that's carried on the back, so it occupies the shield slot and one weapon slot. You can't shield while it's equipped. It does all types of damage (i.e. normal, piercing, elemental and shadow), knocks back, stuns and inflicts poison, shock and fire. Charging it takes ages and you can't move while charging. So it's practically useless without a party that protects you.

The Power Suit occupies an armor slot and 2 or 3 weapon slots. Since it's equipped in place of an armor and has lower defense values than usual 5 star armor, it's powerful attacks are balanced out. Every weapon slot represents a different preset attack move. The attacks are melee attack combos. Weapon slot 1 would be a fast combo attack and weapon slot 2 a slow, but powerful attack. An active Weapon Slot Upgrade would unlock a throw in slot 3, that catches an enemy, stuns it and knocks it back behind the knight. The charge attacks would be a jetfire accelerated forward tackle, a shockwave (that throws back and inflicts shock on enemies and possibly the knight himself) and for the 3rd slot a forward jump that will catch an enemy, crushing it between the arms of the Power Suit.

The Basilisk Helmet occupies the helm slot and one weapon slot. Since it's equipped in place of a helm and has lower defense values than usual 5 star helms, it's powerful attack is balanced out. Its basic attack is a continuous laser beam that does repeatedly a small bit of elemental damage and inflicts stun and sleep. It has no charge attack, because it's continuous, i.e. it fires as long as you hold down the attack button and stops when you release.

From these above examples you would only be able to equip the following combinations, because of slot conflicts:

• Rocket Launcher exclusively
• If you have one Weapons Slot Upgrade in use: Basilisk Helmet and Twin Blades
• Basilisk Helmet and Heavy Battle Axe
• Basilisk Helmet and Ground Shaker
• If you have 2 Weapons Slot Upgrades in use: Basilisk Helmet and Power Suit

Alternatively, if OOO decides to make optionally occupied slots (usually activated through Weapons Slot Upgrades) be replaced by required slots of another Multi Slot Power Weapon, these combinations are valid, too, but with less attacks available and use of one or two Weapons Slot Upgrades:

• Basilisk Helmet and Power Suit, with just 2 attacks of the Power Suit available, but just one Weapon Slot Upgrade in use.
• Basilisk Helmet, Heavy Battle Axe and Power Suit, with just 2 attacks of the Power Suit available.
• Basilisk Helmet, Ground Shaker and Power Suit, with just 2 attacks of the Power Suit available.
• Rocket Launcher and Power Suit, with just 2 attacks of the Power Suit and 2 rocket types of the Rocket Launcher available.
• Twin Blades and Power Suit, with just 2 attacks of the Power Suit available.
• Heavy Battle Axe and Power Suit, with just 2 or 3 attacks of the Power Suit available.
• Ground Shaker and Power Suit, with just 2 or 3 attacks of the Power Suit available.
• Combinations with ordinary weapons, replacing optionally occupied weapon slots, would be possible.