The sequel to Secret of Mana was announced a few months ago. Square has released some screenshots since then, and according to them this seems to be a polygon free action rpg. The backgrounds are hand drawn and are very colourful. I've put up some of the pictures the screenshots section so you can check it out there.
   As for the plotline, nothing is known except it has to do with the mana tree in some way. You can have up to three members in your party at the same time, but i don't know if there is any multiplayer support as in the classic SoM for Snes.
   The Japanese release date has been set to 1999-7-15, but as usual it's still unknown when it will be translated and released in North America and Europe.