If you want totally different sounds then buy a totally different keyboard. Effects boxes can make a big difference in how your keyboard outputs, but in the end, it's still basically just an electric piano with a handful of sounds.
If you need better sounds, then start looking at synthesizer workstations from the big manufacturers. Korg Krome 88, Yamaha MOX8, Kurzweil PC3LE8, all of these will cost you around the $1800 mark. Or perhaps take a look at the new Casio PX-5S which goes for about $1000 even.
If you're willing to live without 88 keys then you should be able to find some cheaper options.
Gary...