According to @evleaks, there’s a mysterious Nokia Lumia 530 (apparently codenamed Nokia Rock) in the making. The handset's model number strongly suggests that it's related to the Lumia 520 and Lumia 525. Since both the 520 and 525 are low-end, cheap devices, we’re expecting the Lumia 530 to have similar characteristics, perhaps featuring a display of around 4 inches.
As you may have heard, Nokia is reportedly also readying the launch of a Lumia 630 / 635, plus an alleged high-end Lumia 930. All these upcoming handsets will most likely run Windows Phone 8.1, which should be released by Microsoft in April.
While the Lumia 630 already appeared in a few images, for now the Lumia 530 and Lumia 930 remain elusive. What we can assume, though, is that - like the 630 - the Lumia 530 and 930 will have on-screen buttons (since Windows Phone 8.1 now supports this).