Agreed with how cheap you can pick up a monitor now as a hatchling sales seem to be ramping up, as Ricky stated since most people don't know how to properly care for them we will see 1 of 2 things in a year or so, a bunch of dead monitors or a bunch of large monitors on CL for sale. (Rant over for now haha)
That being said, Corns, BPs, Leos and Beardies have always been popular, some Boas and Blue Tongues seem to be gaining more popularity as well. It is hard to say what the next big thing will truly be. My guess its whatever a big name breeder starts producing massive quantities of.