1.Code Lyoko. It's a highly popular animated series. There are several things in the show that are easy to turn into LEGO, like Kankrelots. There's going to be three animated films next year, and possibly a 5th Season. If done right, they would sell incredibly well. There are many possibilities for the sets, and making their vehicles, including the Skid, would be simple for the set designers.
2.Super Mario Bros. It's the most popular video game franchise of all time, and knowing the shitload of Mario merchandise they've had in the past, LEGO might as well go for it. Making sets based off of the NES and SNES games would be pretty cool, IMO. I can already tell they'd sell like hotcakes.
3.Kingdom Hearts. It's nearly begging to become LEGO sets. It's a very popular game franchise, with a few games coming out next year, and like Code Lyoko, it'd be simple for the set designers to make, and it'd sell very well.
Legend of Zelda and Super Smash Bros wouldn't be bad, either.
-Rho-