While some might call it a punishment he doesn't deserve to have to suffer another four years as President, how would Lincoln steer the nation after the end of the war and of slavery?
Either he gets the South readmitted under White rule, in which case modern historians will criticise him for not doing more for the Freedmen; or else he will move towards the Radicals position on Reconstruction, adopt a programme more like theirs - and in all likelihood meet the same failure.
Like FDR later, Lincoln died at precisely the right time for his historical image to be at its most positive.
Also, many say Lincoln had some sort of cancerous tumor in his face. This may be untrue, but if it was the case, he'd have been lucky to make it to 1866. Maybe Reconstruction goes a little better, since it isn't being run by a bastard like Andrew Johnson for quite as long, but not much changes overall.