Are there any current Water type Pokemon you'd see likely to gain a new stage in their line in the future generations? If not, are there any you'd like to get one? New stage counts for both pre-evolutions and evolutions.
The first thing that immediately springs to mind, are the bigger one-stage Pokemon like Lapras as Hikari mentioned, Kangaskhan, Bouffalant, where its pre-evolution could be Tauros, and loads more. I also think Pokemon like Spinda and Luvdisc could use some evolutions, where Alumola, which has commonly been mistaken as it, could be the evolution of Luvdisc. Other than that, I can't really think of anything. Possibly middle stage evolutions for some of the fossil Pokemon?