December of 1862 means McClellan has already been replaced
So if you could put anyone in charge of the Army of the Potomac in December 1862 who would you choose? Now I'm not necessarily you have to choose someone from the Eastern theatre but let's try and keep the replacements at least realistic
December of 1862 means McClellan has already been replaced by Burnside, who has since been found wanting after Fredericksburg and the Mud March.
In addition, a lot of solid Army of the Potomac contenders (Kearny, Mansfield, even Porter) are dead or disqualified, and others are unlikely to agree to serve under Hooker, if chosen (as historically).
So, looking at the army as it was for Chancellorsville in the spring of 1863, and working backwards, the available senior commanders were:
Hooker (historically, CG, Army of the Potomac);
JF Reynolds (CG, I Corps)
Couch (CG, II Corps)
Sickles (CG, III Corps)
Meade (CG, V Corps)
Sedgwick (CG, VI Corps)
Howard (CG, XI Corps)
Slocum (CG, XII Corps)
Stoneman (CG, Cavalry Corps)
going back from there to the Fredericksburg battle, the available senior officers are:
Burnside (CG, Army of the Potomac)
Sumner (CG, Right)
Couch (CG, II Corps)
Willcox (CG, IX Corps)
Center (CG, Hooker)
Stoneman (CG, III Corps)
Butterfield (CG, V Corps)
Franklin (CG, Left)
JF Reynolds (CG, I Corps)
WF Smith (CG, VI Corps)
So, in terms of continuity after Fredericksburg and through to Chancellorsville, Hooker had what amounted to an army in any other name at Frederickburg, while his two peers at Fredericksburg, Sumner and Franklin, had departed. Sumner could have stayed, but declined to serve under Hooker - Hooker, presumably, would have served under Sumner, however.
Of the six corps commanders at Fredericksburg, Reynolds, Couch, and Stoneman were the only three to retain corps command by the time of Chancellorsville, and Stoneman was given the cavalry corps to form, so that presumably takes him out of the running...although an Army of the Potomac with a cavalry commander as senior is an intriguing notion (cf Kearny the Magnificent); whether Stoneman was
that cavalry commander is another question, and somebody has to form the Cavalry Corps.
All in all, Hooker was the best available choice in the east in the winter of 1862-63; Sumner, with a solid chief of staff (Humphreys, who was a divisional commander at this time) is one potential alternative. His age would tell against him, but he was a professional to his bones and would be, I expect, above the political questions that had become fairly common in the East by this time. His health would presumably not last, however, which raises the question yet again in 1863.
Other possibilities are Couch, Reynolds, and (of course) Stoneman; all three were capable corps commanders, but Reynolds, apparently, did not accept the command when offered, and it is unclear if Couch and Stoneman were considered for it, historically, and set aside. Reynolds and Stoneman were both regulars in 1861, while Couch was a volunteer.
Halleck, of course, is already GinC, acting as chief of staff, in Washington, by this time.
From the west, by the winter of 1862-63, the available senior commanders (this is after Perryville, so Buell is gone) are:
Grant (CG, Army of the Tennessee)
Corps commanders - Parke, McClernand, Ord, WT Sherman, CC Washburn, McPherson, Banks;
Rosecrans (CG, Army of the Cumberland)
Corps commanders - McCook, Thomas, TL Crittenden
Realistically, the only likely choice from the West at this point, in terms of commanding sucessfully and repeatedly at the army-level is Grant, and he is pretty close to indispensible by the winter of 1862-63 in the West. If Grant is sent east, the Army of the Tennessee has to go to Sherman (although McClernand will presumably make things difficult). The other possibility is Rosecrans, which would move Thomas up to the Army of the Cumberland.
It is an interesting exercise.
Given the realities of the winter of 1862-63, based on experience and seniority, after Fredericksburg I would have approached Sumner for the Army of the Potomac, with the proviso that Humphreys is his chief of staff. If he accepts, plan a very conservative limited offensive, with the minor objective of pushing the front line south of the Rappahanock (crossing somewhere other than at Fredericksburg, of course) and the major objective of tying down Lee's army in Virginia; a strong, Sheridan-like operation in the Shenandoah, under (presumably) Reynolds with a beefed up I Corps and Stoneman's cavalry attached would be another option.
Failing that, Reynolds or Hooker are the best choices in the east.
Best,