According to a job vacancy posted on www.dtf.ru, Nival is hiring a concept artist for "work on AAA project in HMM universe". So unless it is another game in the so popular MMO genre, Heroes VI might already be in development.

UPDATE:
here is full list of vacancies currently open at Nival:

3D graphics programmer
QA Lead
Sound disigner
Lead Animator
Tester
Programmer
Interface Programmer
Server part programmer
Lead Artist
AI programmer
Game mechanics programmer
Tool programmer
Character modeler

All of them note "for project in HMM universe".

(via heroes.ag.ru).