There really should be a seperate tracking of nightmare dragons, so your chance to drop increases with different buckets. You can determine what is 'fair' for max chance drops, and how many required to drop, but this mechanic would greatly increase statisfaction of the playerbase so it isn't completely rng based. . . that there is SOME progression.
To put in my 2 cents, I will just point out some of the results from my runs on the PS3 side.
1. One player I played with received a Hakkon on his first NM run
2. One player I played with received 2 Hakkon in space of 3 runs
3. One player I played with been up for 48 hours in 30+ runs finally got one Hakkon
4. One player I left him still hunting for elusive White Hart he had 40+ runs
5. One player I played with received his second Hakkon after 40+ runs
6. I have one Hakkon after 40+ runs
It seems the "Random" is truly a roll of the 20-sided dice, some deterministic potential will be VERY positive especially to those who did many runs without any drop. Whether this is increased probability in HB mode, or progression based probability, as the suggestions have been made, I am hoping a positive decision is made. Another suggestion is that if the developers truly wants to make the Hakkon weapons and multiple weapons rare, they could allow the player be deterministic in choosing which weapon he receives so that a player not liking rogues class receive a dagger/bow etc.