Based on the trouble he has with routes and the trouble he has with hands I don't think Williams is ready to seriously challenge a likely significantly improved Harris.
Harris showed lots of RAC ability, toughness, blocking ability, decent hands, willingness to go across the middle and perhaps most importantly he seemed to be picking up the offense and making plays.
I think he brings a good dimension to the offense and works well with Dez and Miles who are more downfield threats.
Where Williams may help out this year is running some deep routes to take heat off of Miles and Dez. He can probably do some of the things Laurent Robinson used to do for us in 2011 but don't know if he could develop the same kind of chemistry with Romo. I do think he will add something as a deep threat when he is out there.
I do think the real keys to the passing offense next year are in this order:
1. pass blocking from the OL
2. Getting Dez to pick up where he left off
3. Getting Miles' hammy issues resolved so he can be the force that he showed us in 2009 when he stayed healthy for a full season
4. Integrating Escobar in the 12 set
5. Getting more production out of the #3 WR
I know Laurent was incredible for us in 2011 but not many #4 receiving options ever put up 11 TDs and I doubt we're going to find that guy. It is much more likely to get Dez up to 11400-600 yards and Miles back up to 1200-1300. If you add that to a mediocre running game and that is a lot of offensive firepower.