I was just thinking about that when I came to this suggestion.
I totally agree. Something like the "7 days of space in bag/bank III/IV/etc ticket" but with house energy. Is totally needed, is one (or maybe THE one?) of the only things who are not accessible without diamonds in the account, and that's a shame.
+Wolfbabe is totally right, actually free to play players are an important part of real spending money. Even if they don't actually buy diamonds, their friends do that for them, and in the end the same actual money go to Gameforge.
So please, do it quickly