One thing I’ve thought was missing in Dungeons & Dragons was that no matter how hard the monsters hit you (or vice versa) you would always stand still and occupy the same space. A medium sized creature could hit a small creature and do as much damage as possible, but the small creature wouldn’t budge. For me, that makes the game a little dull.

After several months of very lose thinking about the subject, an idea popped up. The Player’s Handbook speaks of the “Massive Damage” rule (page 145 for v3.5). It says that if a creature suffers 50+ points of damage in a single attack, said creature must make a fortitude save with DC 15. On a failed save the creature dies instantly. My idea regarding knockback can be either used to alter this rule or add to it.

Altering the rule:
This version replaces the fortitude save with a reflex save. The reflex save is made to check for footing. On a failed save the creature is sent hurling back a certain distance depending on the size of the creature doing the damage. A creature of the same size knocks the opposing creature 5 ft. away from itself. Creatures larger than the targeted creature sends it hurling back +5 ft. for every size category that it is larger. So a large creature would knock a medium creature 10 ft. back and a small creature 15 ft. back. A creature of smaller size can not knock a larger creature back but does knock it down. Under most circumstances (see below for exceptions) the creature that is knocked back ends flat on the ground as if successfully tripped. I simply assume that no creature can under normal conditions land standing after such traumatic damage.
Replacing the fortitude save with a reflex save allows all creatures to live longer since they don’t have to worry about failed fortitude save killing them whenever they suffer from massive damage (like that will happen a lot).

Adding to the rule:
This version is much like the other version except it doesn’t replace the fortitude save with a reflex save. Instead it knocks the creature back on both failed and successful saves. The creature is knocked back as far as per the other version, but on a failed save the creature’s corpse is knocked back instead of the actual creature. Failed saves don’t give smaller creatures any bonuses since dead creatures usually end flat on the ground anyway.

If a knocked back creature hits an obstacle before the complete distance, neither versions of the houserule assumes there’s extra damage. Hitting obstacles is pure cinematics and only for show. On the other hand, the DM may rule that sharp walls or other dangerous obstacles can damage the creature even further. In that case, such damage should never be more than 1d4 for sharp walls or 1d6 for something really sharp (like spearheads). To be knocked back to begin with is enough suffering. Being knocked off the surface (such as off a bridge or a cliff) adds the additional falling damage normally.

Since magic is supposed to be all glory and massive, magic does not follow the normal rules for size. Because of how grand magic is made to look, magic has it’s own size depending on caster level. Every spell begins as small size, regardless of the caster’s original size. At 5th level and every five levels after that the spell’s size increases by one. So a 5th level wizard casts a spell treated as medium sized, large at 10th level and so on. A small sized spellcaster at 13th level would thus knock a creature back as if large if he would use magic. Note that this means epic characters of 30th level or higher can cast spells of effective size larger than colossal. Also note that most spells can’t knock back unless the spellcaster reaches a level where the spell is larger than originally. The spell Lightning Bolt, a 3rd level spell, can be cast by a 5th level wizard but can’t deliver massive damage until said wizard has reached 9th level and thus can’t apply knockback until then. At that point the spell has reached medium size and only one level away from reaching large size. This should not be considered a flaw in the houserule. Magic is supposed to be powerful. It’s only logical that when a spell throws someone around it throws him across the room.
Magic always knocks the creature back away from where the spell originated. A spell with a radius knocks the creature straight away from the central point and a spell with a cone effect knocks the creature away from where the cone originated. Some spells simply kill the target, without actually applying damage or just aren’t flashy enough for knockback to fit. DMs should use their own heads when determine which spells do not apply knockback.

Here are some optional feats that can spice up knockbacks:

Powerful Knockback (General, Fighter)
You knock your opponents further away after delivering massive damage.
Prerequisite: Str 15, Base Attack bonus +3
Benefits: When you knock a creature back due to massive damage you are treated as one size category larger than you actually are. If you are small your knockback is as if you were a medium creature. If you are a medium creature your knockback is as if you were a large creature. You can take this feat multiple times, up until your knockbacks are treated as colossal size.
Special: Fighters can take this feat as bonus feat.

Easy Landing (General)
When knocked back you are more likely to land standing than others.
Prerequisite: Balance 5 ranks
Benefits: You are allowed to make a DC 10 reflex save to land standing after being knocked back.

Certain Landing (General)
You always land standing when you are knocked back.
Prerequisite: Easy Landing, Base Reflex save bonus +3
Benefits: You always land standing after being knocked back. This benefit does not apply if you are killed by the attack that knocked you back.

Controlled Knockback (General, Fighter)
You can somewhat control in which direction your opponent flies off to when knocked back.
Prerequisite: Base attack bonus +6
Benefits: When a creature is knocked back from your attack you can control into which direction that creature goes, as long as that direction does not cross your space.
Special: Fighters can take this feat as a bonus feat.

Increased Knockback (Metamagic)
When your spells deliver massive damage, they knock the target further back than usual.
Prerequisite: Caster level 5
Benefits: Using this metamagic feat allows you to treat your spells as one size category higher than it actually does. Applying Increased Knockback to a spell takes up a spell slot two levels higher, so a 3rd level spell with this feat is takes up a 5th level spell slot instead of 3rd.