Realistically, no one worth their salt could replace Putin and expect that person to hold Russia together. Putin made sure no one could upstage him.

The best case scenario for Russia post-Putin is to have a Maduro-type leader. Someone competent enough to hold on to power and enrich his cronies ~boyars~, although could never have the charisma to gain enough popular support and cult following of his predecessor. Thus, the next leader is in a precarious hold on the power like Maduro.