Sproles, Westbrook or LdT are pipe dreams. With 1st round picks in Addai and Brown, a finally healthy Mike Hart working his way into the offense and Chad Simpson showing promising flashes the Colts are set at RB.
Joseph Addai: Addai converted the majority of his detractors this year, though I don't really see what was different than last year beside him being a bit healthier. Addai is a perfect fit with the Colts, he's an elite pass blocker and a very good receiver out of the backfield. As a runner his style means that his 4.4 speed will only make 1-2 appearances a year (he's made 4 plays of over 25 yards in his career), but he excels at making positive plays out of nothing and has a fantastic nose for the endzone. Addai averaged under 4 yards a carry and totaled less than 850 yards rushing, but ranked as the 14th most productive back in the league by FOs numbers due to a top 10 success rate. Addai's value on the ground is in him consistently putting the offense in convertible down/distance situations and solid short yardage running.
Donald Brown: Brown flashed the explosiveness we haven't seen in the Colts running game since a pre-knee reconstruction Edge, but he struggled to stay healthy, didn't seem fully integrated into the offense and lacked the consistency of Addai. He carried the load without issue in college, will have a full offseason of playbook study and work with the team and a bit less consistency is fine if you can rip off the big play, leaving me very optimistic for Brown's future.
Mike Hart: Finally healthy after an ACL tear ended his rookie season early, Hart started to work his way into the offense as the 3rd RB and in the old James Mungro miniature fullback role. Hart's numbers are ugly, 2.7 YPC, under 150 total yards, but this is a case where even I'll say the numbers don't do the play justice. Hart runs hard. Just getting back to the line of scrimmage on many of his runs was impressive with how poorly they are blocked. Hart's no superstar in the making, but he's shown a lot of promise that he could develop into a great role player.
Chad Simpson: Simpson's a puzzle. His performance on kickoff returns was horrific, but when he got in at running back in cleanup duty he put exclamation points on wins. He averaged nearly 7 yards per carry, though it was extremely boom or bust. 70 of his 102 yards came on 3 of his 15 carries. That explosiveness didn't seem to translate to the return game where he had just 1 return over 40 yards in 38 attempts, earning him the nickname around the site of "The Human Touchback".
Likelihood of addition: Low
Simpson's job is the only one that is the least bit in question. The only way I see a running back being taken in the draft is if it is primarily as a returner.