If you are putting in hero abilities such as Spell Power, it needs to effect spells in relation to the opposing hero's SP.
For ex. If your creature's speed is 6 and you cast haste and your SP is 20, your creature's new speed becomes 9. Opponent's SP is 10 and he casts slow, your creature's speed should drop 1/2 of the buff, to 7.5, because his SP is 1/2 of yours.
Nothing frustrated me more in HOMM3 than casting haste first and having slow affecting my units to go far below their normal speed.
Any thoughts of other ways the difference in relationship would be affected?