She's under 50, so she can easily outlive Putin if she isn't murdered.
She's known as "First Lady of the Russian opposition" and is seen as having saved her husband's life when he was poisoned, by demanding he be flown to a German hospital.
And she's always publically criticized the regime, while staying loyal to her husband (which is important in Russia's culture).
She's also got a degree in international business relations.
This just might give progressive groups inside Russia hope for a better future after an unfortunate accident of Putin.