The problem with Beria is that everyone hated him, he could only emerge as leader as a result of a fairly immediate NKVD coup, including the assassination of several prominent generals before they could react. Needless to say, this would be very hard to arrange at short notice.
There's probably be a period of collective leadership rather than the immediate seizure of power by one individiual. The Five Man Committee formed to coordinate the war would probably be the basis of this - IIRC membership was Malenkov, Zhukov, Koniev, Molotov and possible someone else, other than Stalin himself.