I noticed that the "Region" parameter is totally unadapted for BD entries.

It is based on the 6 DVD regions, when they are only 3 for BDs, and they cover different territories:

A: North America, Central America, South America, Japan, Korea, Southeast Asia
B: Europe (EU), Africa, Middle East, New Zealand, Australia
C: China, India, Russia, Rest of the world
(source: http://www.emedialive.com/articles/readarticle.aspx?articleid=11760)

Should it not be wise to add these possibilities to already existing ones?

Maybe with a lock, that would prevent from using them, when contributing a DVD or an HD DVD (and vice-versa).

I am aware that this issue raises another one: what about DVD/HD DVD combos?
At the moment, they should only reflect the region for the DVD part, as HD DVD is not "zoned".

Yet I think it would be preferable to point out somewhere that this concerns only the DVD part, to avoid confusion.

And while writing this, I am thinking about the rumour that it is thought of "zoning" the HD DVD in the future.

It would be prudent to consider this possibility in defining the rules for this parameter, although I keep my fingers crossed that it NEVER happens!