BDKM
Natsu used Blaze dragon mode against Gajeel in the early episodes which said enough. BDKM is either the base Natsu or the weakest mode for Natsu. it is just got upgraded from FDM to FDKM. At Zeref fight Igneel Magic was overwhelming his body he couldn't handle this power that's why he was exhausted no way in hell he could have used Dragon force with Igneel flames that would be where Acnologia is at. Dragon force is the ultimate mode/form or w/e for a dragon slayer. It make sense why he couldn't use DF while using Igneel mana so he used the weakest mode.
Natsu was using blaze Dragon King spells before he revealed Igneel's power many times. Right off the bat, this meant that FDK can be used more than once and wasn't Igneel's power. Happy had no reaction at first. Natsu started unwrapping his bandages when Zeref make his appearance. Happy was surprised when Natsu was willing to use Igneel's magic early. Natsu used Igneel mana and activate BDKM it doesn't mean it is his strongest form. Natsu showed enough feats to put him on top tiers that's my point he ain't even trying but never deemed worthy enough.
To sum up everything regarding his skills just got upgraded.
Fire Dragon king force - Lighting Flame dragon king mode , BDKM etc im not sure but it make sense
Natsu mana.
DF > LDFM > BDKM (base)
Igneel Mana
DF > LDFM > BDKM( base).