Murray is one of the few players to have a winning record against Federer on Hardcourts. Furthermore, none of the younger upcoming players possess the versitility in their game as Murray, whether they will improve this is questionable since its tough to teach.
Murray trained in Barcelona as a teen, so there is no reason why he cant improve his results on clay. Regarding grass, once he can learn to deal with the British media hype at Wimbledon I expect him to be a contender at SW19 since grass still rewards 'flat ball strikers' as demonstrated by Safin runs this year.
Murrays progression to the elite started during the north american hard court swing. The Masters series titles and Grand Slam result at the US Open, prove he can backup wins on the big stage. With this newly found confidence, I expect Murray to make a charge towards the no 1 ranking next yr.