According to CR, there will be two new Canon 24-70mm lenses to be announced in 2014. One is a L-lens with IS version, the other is non-L lens with IS STM version. These two rumored lenses could be released in Mid or late 2014.
Canon EF 24-70mm f/2.8L IS USM lens: This lens would be larger and have optical performance that rivals the current non IS EF 24-70mm f/2.8L II. This IS lens will not replace current lens, just a very expensive bigger brother.
Canon 24-70mm non-L IS STM lens: This one will be the cheapest lens in Canon 24-70mm lineup.