Exactly what it says. You are given total control of 40K... but only for one thing.
You can either change:
a.) One thing about the fluff.
b.) One rule from the game itself.
c.) One rule that any one army uses.
d.) Introduce a new rule.
e.) Any other reasonable "one-off" change.
What do you do, and why?
For my part, I wouldn't make the change for female Space Marines (because that'll happen the day after never).
My change would be to introduce a new rule to the Tyranid codex:
Beta Creature:
Units with the Beta Creature rule: Zoanthrope, Pyrovore, Venomthrope, Lictor, Carnifex Broods comprised of a single Carnifex.
A model with the Beta Creature rule follows all the rules for Independent Characters except where otherwise noted. A unit with a model with the Beta Creature rule attached to it may never use that model's Ld value for any test. The Beta Creature instead uses the Ld value of the unit as though it were one of the primary models comprising that unit (i.e. it uses the highest leadership of the squad, excluding its own. This may mean its Ld value drops).
A unit may have up to three models with the Beta Creature rule attached to it at any one time (i.e. a Brood of them), but may not have different types of model with the Beta Creature rule (so you may attach a Zoanthrope or a Venomthrope, but not both types at the same time). While part of a unit, the Beta Creature follows the Instinctive Behaviour rules of the unit it is attached to.
A unit with a Beta Creature attached may also attach a single model with the Independent Character rule, if there is one available.
Independent Characters may join broods made up of purely Beta Creatures as though they were normal units.