I had an idea for the Yami Yami no Mi that would have made Blackbeard twice as dangerous.
So, like the fight with Ace, right? It turns out there's an advantage to having to absorb every part of an attack. Because then he makes his own version of Fire Fist with all the fire he absorbed into his darkness that burns Ace. Yup, he can absorb materials, and then give his darkness shape, and if it's something from a Logia, it can hurt the enemy user with their own element once it's absorbed. So, like a fight with Kuzan, he could freeze him with enough ice that he absorbs.
Right there, he would have a lot more utility with attacks and be a lot more powerful.