So you're cool with denying gnomes, halfling and other small characters the ability to take a 5-ft step? That's
really stitching up Small races, and for little benefit when it comes to discommoding casters, which we'll come to later.
Not only that, but for some reason you're making the rules dependent upon speed unless they're dependent on size, always to the detriment of smaller races. You're saying that a small, agile, quick-moving creature can't effectively take a step back from a slow-moving behemoth without getting clouted, even if it's on the edge of said creature's melee range. Remember, something being able to duck and weave quickly isn't related to its base speed: no housefly is ever going to beat me at a track event, but I'll be damned if I think I could hit one with a sword. Not out in the open, anyway. Unless I got it drunk first, or something.
And... are you saying that very big creatures have a minimum movement? Because that's absolutely what it looks like when you say things like "10-20' step".
And in the unlikely event that this happens...
"9. Any
creature with a speed of 5 feet or less can’t take a 5-foot step, since moving even 5 feet requires a
move action for such a slow
creature",
...does this...
"6. A given creature's 5-foot step is either 1/6th of that creature's base speed (rounded down to the nearest 5), or as determined by creature size (see rule 7)
whichever is larger."
...apply?
Rule 7 basically says "DM fiat: I say how far a particular creature can step without taking a move action".
You could have just said that to start with and saved yourself a load of typing.
All you're doing here is biasing game balance
even further in the direction of larger creatures: not only do they have crazy reach, but now they can move as far as a standard character without taking an action and without provoking attacks of opportunity. You're claiming verisimilitude, but it's actually not helping "realism" in the slightest: the bigger a creature is, the more ground it covers with a step, to be sure, but it also
takes that step much slower. A 5-ft step might be simplistic, but in a turn-based combat system it is a reasonable amount of movement for something to make without provoking AoO's: a 30-ft step just isn't.
All that aside, you're about to fall heavily afoul of...
The Law of Unintended Consequences
Taking a +1 LA in order to play a size Large creature just got an order of magnitude more attractive in your campaign, not to mention what happens when Large PC's start using Expansion. And what about PC's with enormous base speeds? Monks will multiclass Barbarian and whatever-the-hell-else in order to pick up ever-increasing "step sizes".
And finally, of course, casters have the easiest recourse to both higher speeds
and larger sizes, so they're going to be taking 10'-30' steps with impunity.
Oh, hang on a sec, didn't you consider it an "exploit" for casters to be able to move away five feet in order to cast? Wasn't the point of this exercise - at least in part - "not include allowing abuse by mages, because they should have a high enough concentration to be able to cast on the defensive anyway."?
Oops...
Altissimus said:
You can take a 5 foot step as an immediate action out of your turn, but only in response to an opponent in melee range taking a 5-foot step, and only if you have not already taken a 5-foot step in your preceeding turn.
So, your giant type takes a 15-ft step to the caster in order to dish up a full-attack bashing... but the (size Large or larger, or just massively-buffed-base-speed) caster gets to move away
as an immediate action, without provoking an attack of opportunity. Because, you know, spellcasters really needed the powerup of free, non-AoO-provoking, immediate-action movement.
This isn't a spellcaster nerf, it's a
polymorph upgrade.
Personally, I believe you might want to think this through a little more. Still, it's your game and you can absolutely do as you like
