Is waiting for a D400 realistic?

Yes it is realistic that the D400 will be the next camera Nikon accounces and that it will be released by January 2013.

In 2012 we got mainly FX announcements:
D4 > D800/E > D600 (and the D3200 / J2 in between)

2013 Nikon will fill the gaps in the DX lineup:
D400 > D7100 > D5200 (V2 and J3 in between)

In early 2014 the D3300 will follow which makes the following timeline:
D4 > D800/E > D600 > D400 > D7100 > D5200 > D3300

Don't expect any pro grade DX lenses though. The pro grade lenses for the D400 are the 70-200VR, 200-400VR, etc.. and hopefully the 300/4VR and 100-400VR.