I would simply eliminate cancer. Cells would still occasionally mutate but it would simply be impossible for any type of tumor to evade the hosts immune system for any prolonged time, unless their immune system is in such a bad shape they would have died very very quickly anyway.

They say every living being will eventually develop cancer. It's just a matter of probability. Nowadays, most still don't live long enough, but as medicine progresses and becomes capable of delaying aging and fight more and more other diseases, the role of cancer will become worse and worse. In a hypothetical perfect world were all other diseases were stopped and no one would age, cancer would be responsible for 100% of the deaths, and still everyone would die.

Therefore eliminating cancer will free life from its greatest nemesis.

Also, it'd be awesome! We could finally have a proper nuclear war without having to fear those nasty long term repercussions...