I guess it would make sense for me if Valve skips this next year, since they have nothing officially announced. I think they would probably announce something (HL3, LfD3) next year and release it in 15'.
Apparently, Valve has released at least a game each year since HL debut in 98, except for the year of 2002:
98 - HL
99 - TF, HL Opposing Force, HL Uplink
00 - CS, Ricochet, Deathmatch Classic
01 - HL Blue Shift, HL Decay
02 - *none*
03- DoD
04- HL2, CS Condition Zero, CS Source
05- DoD Source...