The short answer to the question, "Can Roger Federer win another grand slam title or ever again be year-end No. 1?" is "No."
The long answer is set out below.
The above question can be split into three questions:
1) Can Roger Federer ever be No. 1 again?
Answer: Yes, it is possible for Federer to be No. 1 for a brief period after Wimbledon. Nadal has a humongous number of points to defend, and Federer has very few. However, Djokovic is ahead of Federer, and he, too, has very few points to defend.
But, in the past, this has not been the best part of the season for him and Federer has won the French Open once and Wimbledon six times. Djokovic has, however, improved his serve, fitness and mental fortitude. So without seeing how they play in the next few tournaments, we can say that both of them are equally likely to displace Nadal from the top.
2) Can Roger Federer win any more grand slam titles?
Ans: Highly unlikely.
In the Open era, out of the 172 grand slam titles won so far, only 21 titles have been won by 11 players when they were older than Federer (29 years, 9 months, 27 days on June 5, the date of the next grand slam final).
The players were Ken Rosewall (four), Rod Laver (five), Jimmy Connors (three), Andre Agassi (two) and Andres Gimeno, Petr Korda, Pete Sampras, Arthur Ashe, John Newcombe, Andres Gomez and Ivan Lendl (one each). Out of these, 18 titles were won by 10 of these players when they were 30 or more. Lendl did not win any after 30.
Federer's chances have been estimated optimistically by observers, comparing him with Connors, who won two titles, Agassi, who also won two, and Sampras, who won one after crossing 30.
Federer isn't like Connors, who was a tough fighter. Federer is more passive and has been seen to go to pieces, even in his prime, when faced with an equal who did not roll over and die.
As for Agassi and Sampras winning titles at relatively advanced ages, we have to remember that they won those titles in the "tween" years when the aging maestros were in decline but a true challenger hadn't risen till Federer started his reign in July 2003.
A similar serendipitous situation does not exist today. Federer has Nadal, Djokovic, Murray and Del Potro to contend with. We cannot also overlook Soderling, Berdych and Ferrer, who have proved capable of upsetting top seeds.
3) Can Roger Federer ever again be year-end No. 1?
Ans: Almost impossible.
If we look at the database of year-end No. 1s since 1973, when the computerized ranking system was introduced, we find that no one aged 30 or above has ever been year-end No. 1. So, statistically speaking, the chances of Federer ever again being year-end No. 1 is almost zero.
Even without looking at the database, it does seem rather unlikely that Federer will accumulate 11,000-plus points this year when there are players like Nadal, Djokovic, Murray, Del Po, Soderling, Berdych and Ferrer, not to mention young up and coming players like Raonic, Tomic, Dimitrov, etc.
Federer has been blessed by the stars with outrageous talent and outrageous luck. So who knows, his luck could make several low probability events happen resulting in his winning a slam or two, but I doubt whether any alignment of stars could ever make Federer be year-end No. 1 this year or any time in the future. So Sampras' record of being year-end No. 1 for six years is likely to stand for the foreseeable future.