Reverse gear would be cool.Yami Yami would be useless and Luffy has all the haki hax to beat BB,he would just need to Bloom his haki enought to compete with Gura AP.
Haha yeah! It would make great training for Blackbeards nullification powers. Ive expected Blackbeard will be the classic case of relying too much on your DF, especially with him having multiple ones ofc and thus Luffy will overwhelm him with sheer haki supremacy that Shanks will help him fully master beforehand too.
I even expected adv CoC could potentially nullify DFs temporarily and force opponents to fight more equally and like "true pirates" or warriors. This all depending on how you perceive pirates though ofc.
I really want a simple brutal brawl at some point between Luffy and Blackbeard, after a ton of absolutely DF insanity has inevitably happened ofc.
I definitely see Luffys DF being vital vs the Gura too. Maybe Luffy will become all 3 types of DFs to match Blackbeards most likely 3 DFs and counter each DF in his own ways.