I hope that one of the big deaths isn't Ando, but if it is, as long as it has something to do with Hiro becoming a better Hero out of it, its cool. Like in a sense to dedicate himself in memory of Ando.
Also, I think Issac may turn dark out of all of this. I think it may be due to the other huge death, Simone, and him blaming it on Peter.
Also, about the visions, I think Peter got the ability from someone else. Part of me thinks that may have been his special connection to Simone's father. He had some sort of power, IMHO, why not be the ability to see the future? He was rich after all
With all that said, I think, and hope, that the "big" deaths are going to be Ando (hate to say it, but this could lead Hiro into fully developing his powers, as Ando has been the only one that really believed in him). And the other one, Mr. Bennet (assuming he really isn't Linderman, I don't think the Linderman character has been cast like Ryan said, but that doesn't mean it won't end up being Mr. Bennet). He's huge in the sense that with him gone, no one will stop Claire from getting out of Texas and finding Peter. My guess is he may be the March 12th death and trigger Claire's "running away" to find Peter.
I chose Peter, as I am pretty sure he's the only other Hero she has come into contact, that she knows for sure is a Hero.