Marshall isn't foreign to the slot position. According to Pro Football Focus (subscription required), he played there 22 percent of the time last season, but that could more than double next year if new offensive coordinator Aaron Kromer brings with him the playbook he had with the Saints.
According to PFF (subscription required), Marques Colston—who is nearly as big as Marshall—played in the slot 55.7 percent of the time last season. While there, he caught 63.1 percent of the passes thrown his way—nearly half of his total for the season—and half of his 10 touchdowns. He averaged 1.61 yards per route run in the slot, the ninth-best mark in the league of all the receivers who played more than 50 percent of their snaps in the slot.
The obvious reason for this was to create mismatches. Above you see a play against the Packers in the Saints' Week 4 meeting this past season.
The play isn't very creative, but it got Colston single coverage. When the Saints moved their fullback, Jed Collins, out wide, Green Bay had to react by putting cornerback Sam Shields on him. The result was Colston had single coverage with safety Morgan Burnett, who he beat—with a little push off—for a touchdown.
It was simple, but effective. Brees threw the ball up and Colston came down with it. the same kind of passes Marshall and Jay Cutler executed last season, but the play design made the receiver's job easier.
There is no reason to think that if the Saints can do that with Colston, the Bears shouldn't be even more effective with Marshall.
Marshall was good when used in the slot last season. He caught 66.7 percent of his passes thrown his way while there. His 2.65 yards per route run in the slot was 16th among all players, according to PFF (subscription required).
It wasn't always that easy for the Saints. In the above play, you see Colston in the slot working the middle of the field. This time the Packers had him matched up with a cornerback with safety help over top, but because of how they were lined up, the safety couldn't get there in time.
The Saints had tight end Jimmy Graham lined up at the bottom of the screen and Darren Sproles flanked to the left in the backfield. Green Bay had to pay attention to those two with the safety on that side of the field.
To the right of Colston, the Saints had Lance Moore and Joe Morgan—who already had an 80-yard touchdown catch in the game. Essentially, Burnett had to decide whether to help over the top of Moore, Morgan or Colston. While he hesitated, Brees threw strike to Colston for a 27-yard gain on third-and-14.
It's a play the Bears couldn't have run last season, but, the addition of Martellus Bennett—who could certainly fill Graham's role in that play—the Bears' playbook should be thicker this season.
The most effective changes the Bears make to their offense might be the most simple, such as finding new ways to get Marshall and running back Matt Forte the ball. That's just one way the Bears can improve their offensive production next season.