I am not going to speculate on who could be taking over the program (besides, others have already covered it and done it well, so all I would really add is random regurgitation of the same facts). But it is worth noting that that when Williams first floated the idea of retiring (last year and then again right after the season), the university was so keen on keeping him around that they put together a contract extension. So in no way should this be read the same way as the Friedgen ouster, because Gary was welcome to stay as long as he pleased.
In 22 years, Gary Williams' teams went to 14 NCAA tournaments, 5 Sweet Sixteens, 2 Final Fours, and 1 National Championship. And he did it after taking over a team that was in shambles after the failed tenure of Bob Wade (who left the school in hot water w/the NCAA, leading to the team being banned from postseason play in 1991 and 1992 and NO TV coverage during the 1990-91 season). Add to that the facts that at the time of his retirement he was the 5th winningest active coach, he was the 3rd winningest coach in the ACC (behind only Dean Smith and Mike Krzyzewski), and until the 2010-11 season the Terps were the 2nd winningest team in the ACC (behind Duke), and he did all of this without a single whiff of a hint of impropriety or scandal (compare that to Jim Calhoun, who just won his 3rd NCAA Championship, but was just hit with a 3 game coaching suspension for the beginning of the 2011-12 Big East season, recruiting and scholarship restrictions).
Add all of that up, and you get a man who more than earned the right to go out on his own terms and in his own way. And a man who probably cannot be replaced. I hope you have enjoyed watching the Terps play basketball for the last 2 decades, because no matter how well they do under their new coach, it will never be like it was in Garyland.