McClellan shifted Morell to reinforce Burnside on the proviso this was a temporary reinforcement to support him if he were attacked. Burnside used the division to relieve his battleline, putting it west of the Antietam and thus stymieing any thoughts McClellan had about using them in the centre. Couch's division (maybe 5,000 effective infantry) arrived midmorning on the 18th, and was sent to Franklin, where two brigades formed left of Slocum, and another relieved Irwin's Brigade of Smith. Finally Humphreys arrived with his exhausted raw recruits (at most 5,000, and probably less) which McClellan used to try and wrestle Morell back from Burnside - and Burnside promptly put them into his reserve and didn't send Morell back!
With all this in mind, and with many of McClellan's Corps commanders objecting to renewing attack, especially Burnside and Sumner, what was McClellan supposed to do? The only option I can see, given his situation, is to launch an afternoon attack by Franklin on the Dunker Church again. If McClellan manages to get Morell back from Burnside (which might mean relieving Burnside of command) he might get 14,500 effective infantry from 5th, 6th and 12th Corps into action there. However that's essentially just a replay of the assault by 1st and 12th Corps and Sedgwick's Division (2nd Corps) assault of the late morning on the 17th, presumably with similar chances of success.
Campaign wise, if McClellan orders such an assault and it fails his army, and the Union, is in dire straits. Whilst Lee likely won't be able to counterattack, if he moves as he historically did then McClellan won't have fresh forces to block Lee's reentry into Maryland via Williamsport. The campaign may well continue with Lee conducting a turning movement and regaining the advantage, or maybe Lee's troops were too exhausted. Either way the risk must be weighed against the benefits.