The Northern Plaza

The Northern Plaza would have consisted of:
The Cultural Congress Hall (Haus de Kultur)
Exhibition Centre
Two obelisks (intended to frame the view of the Great Avenue)

The rear and side entrances to the new Congress Hall would have backed onto this area making it extremely busy.

The area is marked on Speer's model of the Rally Grounds which went on display at the 1937 Paris World Fair

There are no images of exactly what was planned but it is believed that part of the design would be to hide the fact that the Luitpold Arena was out of alignment. 
There was some consideration to rotating the arena to match the symmetry of the new structures (Congress Hall, March Field, Great Avenue and German Stadium) but this was a dream for later times.

The Haus of Kultur and Exhibition Hall would have provided an imposing entrance portal to the rally grounds.

Possibly they were intended to serve functional purposes related to the massive events, likely housing logistical support, offices, or possibly actual exhibition spaces related to Nazi ideology, but details of their planned use is sparse.