According to latest rumors, Nikon D610 coming with a minor improvement in Top continuous shooting speed (5.5 vs 6fps) , This look like a software tweak done with Nikon D600 firmware, nothing new in D610… however Nikon D5300 is coming with a new Expeed 4 image processor, that will sure show major improvement in the core specification.
Now about Pentax, Pentax latest K-3 DSLR is expected to be announced at the same date, the Pentax K-3 will feature new sensor possibly made by Sony, Electronic AA filter and new image processor, see more about Pentax K-3
Main upgrade with Nikon D610 will be new shutter – to avoid dust on sensor what was a huge problem with D600.