So, on one side, we got Luffy who has the power created from the hopes of freedom from tyranny. And then on the other side, Blackbeard, who has a power made from the desire to consume the world in darkness and nullifies other's hopes.
Joy vs Despair or even Nihilism, interesting.
I do think BB will try to spread a literal and metaphorical darkness across the entire world, with his Yami awakening especially and thus make it permanently that way too.
Also trying to force everyone to sleep forever so they can never stop dreaming (Infinite Tsukiyomi and Uta style) or preferably, he inflicts insomnia on everyone purposely as he apparently can't sleep for sone reason and may want them to suffer as he does too?
I see Blackbeard and Xebec as the dark side of freedom too, the extremists, who take freedom to do anything, too literally, too far.
True lawlessness (pun not intended in this ironic situation with him and Law lol).
BB has a move called Liberation too after all...
Blackbeard being like a black hole thematically too is great though, a walking void that nullifies all hope, joy, dreams, ambitions etc. Just drains you of all good things and encourages despair.