*** SPOILERS ***
Those who heard the datamined sound files and Uther's dialog will understand the necessity.
IMO, the first three—Bolvar, Tirion and Mograine, in that order—are the most likely candidates. The others are there for variety. Saurfang Jr. is confirmed dead (permanently this time?), so not listed.
Muradin has already had an encounter with Frostmourne. Highly doubful of Saurfang.
Jaina / Sylvanas...The Lich Queen?
Thrall, since it seems almost confirmed that he's leaving the Horde. Some have claimed that Med'an will be the new Guardian, so that would leave Thrall out of a job. Controlling the Scourge might become a nice hobby for him.
Varian is listed only because Thrall is...
HR_Uther_QuelDelar06 - I can resist Frostmourne's call no more.
Oh, and I this one has been trumpeted by some in the forums, and now confirmed by Uther: Arthas is still alive in there somewhere. Might that remnant of him (aliased Matthias Lehner?) continue to keep the Scourge in check?
Any options below Arthas is facetious.