Nokia probably is working on another Asha model called Asha 502, this is still in the rumor phase and not yet confirmed. Evleaks however has leaked images of a new Nokia model what they call as the next Asha, 502.
What we know for now is that the Nokia Asha 502 will be dual SIM and Nokia might get the 3G capabilities on this handset. It will be an upgraded version of Nokia Asha 501 and they might add some features that were missing on the 501 handset.
If rumors are to be believed Nokia is also working on more variants of 5xx series namely 500, 502 and 503. Well not much of the info are known, it would be wise to wait for the official launch that might happen with few weeks or months.