I would think that Whirlwind attack and Cleave would not be usable with this. It seems that they are geared to staying in the same place.
With the Dragon you could say it could substitute any of its attacks as movement instead of the entire iterative.
I was thinking of augmenting the 5' step as follows:
Combat Shift:
Base speed 5’ no 5' shift
Base speed 10’ – 30’ 5' shift
Base speed 35’ – 60’ 10' shift
Base speed 65’ – 90’ 15' shift
etc.
This would be augmented by size (based on the occupied square size)
5' Tiny - Medium +0'
10' Large +5'
15' Huge +10'
20' Gargantuan +15'
30' Colossal +25'
This total cannot exceed the character/creatures base move.
Any movement over this value would provoke an Attack of Opportunity.
This movement could be divided in half and taken between attacks. if the move ends in a 5 then it the 5’ would be chosen between the first or second move.
i.e a character/creature w 5’ shift could only move 1 time
10’ could move 5’ and 5’
15’ could move 10’ and 5’
etc
This would allow a creature that is Colossal move a normal steps distance. A 5-6’ human covers about 2.5-3’ with a step (about half its height) so a 5’ step is really about 2 steps. So a 30’ tall giant would cover 6 times that which would be 30’ (2 15’ steps). This would be more in line unless it has really tiny legs. A 5’ step for a colossal dragon means it would have to take baby steps the equivalent of a human taking about a 5” step. Sort of unfair.
Second it allows for characters/creatures that move with exceptional speed to move a bit farther without being attacked.
This would also make feats like Combat reflexes, Step up, and Mobility a bit more attractive. I believe it would also augment the desire to take Shot on the run or Spring attack to keep up with the pace.
You could also use this with the feat system mentioned earlier.