In its typical last-minute fashion, FIDE is bidding out an eight-player double round-robin Candidates Tournament to choose Carlsen’s challenger next March. Summary on Wikipedia.
Three players have clinched a spot: Anand (as loser of the 2014 championship match), and our own Caruana and Nakamura (as Grand Prix qualifiers). Two other players will clinch the two World Cup spots shortly: the winner of the Giri-Svidler and Karjakin-Eljanov semifinals.
The top two rated players not otherwise qualified (based on average of monthly FIDE ratings) get an invite, too. Topalov and Giri have virtually clinched spots anyway based on their wide lead in 2015 average rating (check my math 
). October 2015 is of course projected and November & December are projected based on October. One has to win a lot of ELO in a hurry to move one’s average rating for the entire year by two points.
If Giri beats Svidler, then either Kramnik or Grischuk is most likely to take the third spot.
[size=55] Topalov Giri Kramnik Grischuk Aronian So Ding Liren
Dec 2,802.5 2,795.7 2,779.2 2,751.9 2,784.1 2,761.7 2,780.6
Nov 2,802.5 2,795.7 2,779.2 2,751.9 2,784.1 2,761.7 2,780.6
Oct 2,802.5 2,795.7 2,779.2 2,751.9 2,784.1 2,761.7 2,780.6
Sep 2,816.0 2,793.0 2,777.0 2,771.0 2,765.0 2,773.0 2,782.0
Aug 2,816.0 2,793.0 2,777.0 2,771.0 2,765.0 2,779.0 2,770.0
Jul 2,816.0 2,791.0 2,783.0 2,771.0 2,765.0 2,780.0 2,749.0
Jun 2,798.0 2,773.0 2,783.0 2,781.0 2,780.0 2,778.0 2,749.0
May 2,798.0 2,776.0 2,777.0 2,780.0 2,776.0 2,778.0 2,757.0
Apr 2,798.0 2,790.0 2,783.0 2,794.0 2,770.0 2,788.0 2,751.0
Mar 2,798.0 2,790.0 2,783.0 2,794.0 2,770.0 2,788.0 2,755.0
Feb 2,800.0 2,797.0 2,783.0 2,810.0 2,777.0 2,788.0 2,755.0
Jan 2,800.0 2,784.0 2,783.0 2,810.0 2,797.0 2,762.0 2,732.0
Proj avg 2,803.958 2,789.508 2,780.550 2,778.142 2,776.442 2,774.925 2,761.817[/size]
Here’s a summary with less eyestrain.
Topalov
2,803.958
Giri
2,789.508
Kramnik
2,780.550
Grischuk
2,778.142
Aronian
2,776.442
So
2,774.925
Ding Liren
2,761.817
Soap-opera complications: if the event is held in Russia, is it possible that Topalov would choose not to play? (He’s made noises about this issue several times.) And Kramnik might turn down an invitation if Grischuk is next in line.
If Svidler beats Giri, we could be looking at a Candidates tournament without Kramnik, Aronian, or Grischuk. There is an organizer’s spot to be filled.