Hideo Kojima doesn't rule out the possibility of making a Metal Gear Solid 5 game even though he previously said he was done with the series and that MGS 4 would be the last in the Solid storyline.
Original plan was to let another person take over the series but in a new direction or storyline while he would only give suggestions or help if the new team needed him (similar to Miyamoto being used as an adviser for Metroid Prime series).
http://kotaku.com/5641618/hideo-koji...l-gear-solid-5