Changes on the defense

The complete lack of a pass rush turned the top pass and scoring D in the league into one that gives up 264 yards and 3 TDs to Marmalard.

This has caused some questioning of the Colts style of defense, we should blitz more, mix up packages, etc. My response to this is, "The enemy of good is better," and if your defining #1 pass and scoring D as good, there's really not a whole lot of room for better, only worse. There is nothing wrong with the Colts defense that requires a change in scheme.
Freeney's return and Mathis and Brock getting healthy should do a lot to fix the pass rush, but a definite weakness in the Colts D was revealed this year, there are only two players on the defense that can get pressure from the outside. Polian needs to dust off some second day magic and find the Colts D another 2004-05 Robert Mathis, a situational pass rusher who can come in on passing downs and backup both DE position. Mathis in 2004 and 2005: 22 sacks, 1 start. Mathis fell to the fifth round in 2003, there are guys like him out there and the Colts need (another) one.
The other question on the D-line is whether McFarland should return or be cut for cap savings. Ed Johnson became one of a small group of undrafted players that started every game of their rookie season. Has he made McFarland unneeded and not worth his cap number? Well, the D was amazing all season with Johnson in the middle, but the Colts D with McFarland won them a Superbowl. On this one I have to answer "I don't know." If cap room is tight he can be done without, but keeping him would be great if possible.
Nickelback was the other sore spot on the defense. Tim Jennings was consistently out of position and committing penalties. Some of that is expected from a second year player who is asked to play on the outside instead of the slot and sometimes cover the opposition's best receiver (honestly, why did they put him on Moss, that was the dumbest thing I saw from the Colts all year). Rookie Dante Hughes looked strong late in the year and saw some time in the nickel package until a shoulder injury landed him on IR. He could take the nickel spot next year, he is basically Marlin Jackson Lite. That's good and bad. Jackson is a great cover two corner, but the Colts like to move him to the slot to hide his lack of speed. Hughes has the same problem, only one of them can play inside in the nickel leaving one on the outside in a mid to deep passing situation where they're are at their weakest, that's why a nickelback who can play on the outside is still needed. Polian loves drafting DBs so there should be a whole new batch of rookies to get a shot.
The linebacker position is as solid as it's ever been. Brackett is performing at a Pro Bowl level, Keiaho is the beast we thought he would be and Hagler grew into his role on the strong side. Rob Morris will be healthy and is under contract for next year and was playing well before his injury. Colts rookie linebackers might as well be in the witness protection program, because they are drafted and disappear for two years, but Clint Sessions found his way onto the field and was excellent in his limited playing time. The linebacker position appears to have recovered from the talent drought that allow Gilbert Gardner into the starting lineup.