There's an interesting book about Lincoln's planned Reconstruction policies: William C. Harris, _With Charity for All: Lincoln and the Restoration of the Union_ (University Press of Kentucky 1997). In the last chapter, he goes into explicitly counterfactual history: what if Lincoln hadn't been shot?
He concludes that while Lincoln favored relatively lenient terms for Southern whites, and was not about to impose black suffrage as a condition for re-entry into the Union, nevertheless there was a difference between the policies Lincoln would have followed and those Andrew Johnson followed in OTL (p. 269):
"[Lincoln's] exalted standing with southern Unionists and his experience in dealing with them to achieve his purposes (for example, the dramatic acceptance of emancipation by many formerly proslavery Unionists) would have produced changes in the South different from those that occurred under Johnson's adminstration. Lincoln...would have paid closer attention than Johnson to the postwar plight of the freed blacks and white Unionists. His influence on the side of bona fide freedom for blacks would have prevented the kind of racially discriminatory laws, or Black Codes, enacted by several of the Southern state governments after the war--laws that Johnson implicitly endorsed...Though Lincoln had demonstrated his willingness to let bygones be bygones, he would have made clear his opposition to an early return of rebel leaders to political power, a position he had expressed during the war. Such a stance would have prevented the rash pardoning that occurred under Johnson during the summer and fall of 1865, arousing the Republican majority in Congress against the new President." Harris thinks it is inconceivable that Lincoln would have shown Johnson's lack of leadership, which he blames for such outrages against blacks and white Unionists as the New Orleans riot of 1866.
I think Harris puts too much emphasis on Lincoln's plans during the war, and not enough on how stubborn southern resistance to black rights might have "radicalized" Lincoln after the war, as it did many previously "moderate" Republicans. He does at least mention the possibility on p. 275:
"nforeseeable contingencies, such as terror campaigns to undermine black freedom and loyal control, might have compelled him to adjust his Southern policy to meet new realities." It should be noted that Lincoln by 1865 had no objection to black suffrage--he stated that he wished the Louisiana government had permitted some blacks (e.g., those who could meet educational requirements or had served in the Union army) to vote. At that stage, however, he was not ready to *insist* on it. I think it is quite conceivable that Southern resistance to black rights would eventually have driven him to such an insistence.
There would of course be no impeachment in any event--Johnson was totally out of touch with Northern public opinion, and drove radical and conservative Republicans together into opposition to him. That would never have happened with Lincoln.