Michael Calderone's got it:
Tyndall said that if he were NBC News President Steve Capus, a short list for the position would include White House correspondent David Gregory, chief foreign affairs correspondent Andrea Mitchell — both of whom have guest-hosted “Meet the Press” — as well as political director Chuck Todd and “Hardball” host Chris Matthews. Two dark-horse candidates could be “Morning Joe” host Joe Scarborough or perhaps former “Nightly News” anchor Tom Brokaw — that is, if he had any interest in returning to such a prominent role.
Matthews seems like the most likely choice to me, since for several years now in addition to Hardball he's been doing a more staid Chris Matthews Show on Sunday mornings that appears to me to have been training wheels for eventually stepping into Russert's shoes. I'd say the best choice would probably be Chuck Todd, who based on his on-camera work during primary evenings would bring a different approach rather than trying to do what Russert did and not doing it as well.
UPDATE: Of course you could really shake things up by going outside the box with Mélissa Theuriau but there's no evidence she speaks English. Closer to the box, several commenters have suggested Gwen Ifill who would be good if NBC is willing to hire someone from outside the network.