Kramnik plays 1.e4, 1.d4 and 1.Nf3 with White. Many think that he played more convincingly with 1.d4 and 1.Nf3 then with 1.e4, however he has played 1.e4 most of the time in recent years, until his return from illness. At that point it was already clear (or at least very possible) that he would play the match against Topalov, so he could well have been “bluffing”.
Against 1.e4 Topalov defends with the Najdorf if he wants to play for a win with Black, sometimes he varies with the Sveshnikov. If he thinks a draw is enough, he plays the Berlin.
If Kramnik would play 1.e4, I think Topalov would go for the Najdorf, unless he has a big lead in the match. He beat Kramnik twice with this sharp line in 2005, and Kramnik did look very bad indeed in these two games. Also Kramnik plays the Berlin and the Sveshnikov himself, so even with a lead Topalov would maybe not venture these lines, and switch to another safe opening, maybe the Marshall.
Therefore I think Kramnik is going to avoid these sharp lines, unless he is behind and desperately needs to change.
Against 1.d4 and 1.Nf3 Topalov used to play sharp openings like the Benoni, the King’s Indian or even the Volga gambit, however in recent years he has switched to more solid systems like the Slav and the Nimzo Indian. I think Topalov would not risk the Benoni or the King’s Indian, unless he is behind in the match and thinks he has to win with Black by all means.
If White plays 1.Nf3 or 1.d4 Nf6 2.c4 e6 3.Nf3 Topalov usually does not go for the Queen’s Indian, but plays 3…d5 instead. Kramnik could then play some form of Queen’s gambit, or the Catalan.
Rublevsky is a second of Kramnik during this match. Rublevsky always plays 1.e4. Against the Najdorf, he likes to play 1.e4 c5 2.Nf3 d6 3.Bb5+. This could mean that Kramnik has been training this variation with Rublevsky, and wants to avoid the sharp lines at move three instead of at move one. Topalov had to face this line a couple of times through his career, and played all the moves 3..Nc6, 3..Nd7, and 3..Bd7. In recent years he had to face this line twice against Kasimdzanov and Morozevich, and both times played the solid 3..Bd7. The game against Morozevich was at the San Luis world championship, Topalov won the game and the tournament.
Other seconds are Motylev (also always plays 1.e4, but the sharp Najdorf main lines that I think Kramnik is going to avoid) and Illescas (who plays many openings).
So what do I think will they play in the first game?
1.e4 c5 2.Nf3 d6 3.Bb5+ Bd7 4.Bxd7 Qxd7 and then White will try to establish a centre with c4 or c3 and d4, or
Topalov may anticipate this and prefer the Marshall, or
1.d4 Nf6 2.c4 e6 3.Nf3 d5 4.g3, or
1.d4 d5 2.c4 c6 3.Nf3 Nf6 4.Nc3 a6.