Yes you can be arrested for conspiring to commit a crime in the future. This is a murky area of law in England, as a group of teenagers who were recently tried for conspiracy to commit murder were acquitted. They wrote about copying the Columbine massacre in America, but the court's view was that it was simply mere fantasy. A muslim woman was also cleared of this charge after she wrote a poem about a suicide bombing. Clearly we can't prosecute someone for writing a poem or a novel about a crime.
But if you publicly stated your intent to commit a crime now, and later actually carried it out, that statement would be good evidence of forward planning, a key element of a murder charge.