Originally Posted by Endeavor
There's a possibility there that it could be made to work, but keep in mind that the secondary mutation caused by Beast's serum was related to the other mutant's power/mutation (blue pigmentation/ physiology alteration).
Leech's power/mutation is to be an inhibitor, not a catalyst.
For Rogue to get closer to the comic books, and be a bit more useful in battle, she has to have more power displays.
There are only a few scenarios.
If the film is set after X3 (in which she was cured), either the effect of the cure wore off, or it triggered a secondary mutation.
To me, it seems easier to believe that the cure was less effective with some mutants. Since Rogue's power takes the lifeforce of other beings on physical contact, her own biochemistry would be trying to absorb the power of Leech's DNA cure serum at the same time as the Leech serum was trying to cure her mutation.
The overall effect would initially be that her power and the cure cancelled each other out. But the serum doesn't increase in any way, whereas her body does naturally regenerate and replenish itself. So, over time, her power wins over the cure.