My guesses wouldn't actually be guesses... they'd be more of "highly likely" candidates based on knowing some prophecies that GRRM has set up in ASOIAF.
In other words, there are some characters slated to die per prophecy stories withIN the books.
[In simpler terms, as of end of last book the characters are alive... but it's just a matter of time since all GRRM prophecies do come true - and this one is a clear interpretation.]
I could reveal, but I think we'd need to put a SPOILER alert.
For anybody that does want to spoil it for themselves, I believe two characters would be mentioned in this linked prophecy:
http://www.westeros.org/Citadel/Prophecies/Entry/1827
I can't say for sure if it'll be THIS season... but they aren't highly main characters so they could easily be written out before the book does them out.
And the third character (being MAIN) is too important for the show at this point and will not be killed as long as that person's actual HBO contract allows...
possibly being killed off in their last season's Episode 9/10 (since Ep.9 is the new "shock" value ep).