Here are some options Grant and Sherman could have looked at:
1. As
@rbasin suggested, land at City Point and extend the Petersburg line to the west.
2. Land the army in Morehead City and push west to Goldsboro, Raleigh and Greensboro. This would jump the army 350 miles north from Savannah and put it behind the small force Beauregard was building in western South Carolina. The Atlantic & North Carolina RR was already in Union control and operation for the first 50 miles of the trip to Greensboro. Historically, it was upgraded and became one of Sherman's supply lines as he marched north. Objective: end the supply line to Lee from Greensboro and remove Greensboro as a rally point if Lee managed to retreat from Richmond.
3. Land at City Point, but remain a mobile force -- do not join the trench war. Stay south of the siege operation and head west. When well past the end of the lines, turn north-west, toward a point on the Richmond & Danville RR. This would cut the South Side RR on the way and would probably put Sherman's 60,000 troops on the retreat route that Lee wanted to use. Therefore, Lee is surrounded, end of war.
4. Land at Harrison's Landing, which Grant had restored earlier, head north to the Williamsburg Road and go west to Richmond. This would outflank the Deep Bottom position and stretch Lee's line so far it could not possibly have held. Sherman's troops were not yet unwilling to attack fortifications, so they would only be stopped by a real force. Distance for this route is about 35 miles to Richmond. This gives Sherman the ability to maneuver as he sees fit.
5. Land at West Point and head west to Richmond on the Williamsburg Road. About 35 miles again and all the same benefits of #4.
6. Land at Tappahannock and take the Mechanicsville Turnpike southwest into Richmond. About 45 miles and the same benefits of #4.
I think any of these would have been beyond Lee's ability to counter. #2 and # 3 had the benefit of preventing Lee from retreating south. All but #1 would let Sherman control a mobile force, outside of the AOP. I believe all possibilities are logistically feasible, considering Lee's very weakened condition.