Lingris

I think it is already a fact, that Loki will be the next strawhead.

When ether Luffy asked some one to join the crew, that person first said no but joined later.

Only Robin is kinda an exception (at least until water 7).

Zoro, Ussop, Sanji, Nami, Chopper, Franki, Brook and even Jimbei: They all said no at first.

Yeah, I believe that his DARK DARK Fruit is a hidden god level zoan fruit, just like Luffies.

That might be the reason he hides himself while stealing the tremor fruit.

It would also explain why he can't turn into darkness.

I think Blackbeard will get a Logia type fruit as his third one. Or he already has a third one.

Hmm, I think he has some powers related to his eyes.

I have to reasons to believe that:

a) He is blindfolded.

B) He doesn't look like someone, drained from sea stones.

Maybe he needs eye contact to inflinct his powers.

He is called "Loki" for a reason. Maybe he has the power to let everyone believe in his lies or he can become/transform into the dream of someone.

I think, that will be the catalyst to set him free:

Luffy lifts the blindfold and Loki can use his powers to make Luffy free him.

If his chains are made out of sea stone, Luffy cannot free him.

So Lokis powers cannot be as simple as to just transform his body. Otherwise he could just free himself.

If he has a logia, it will be a more complex type, like Blackbeards or Ryokugyu's Woods Woods Fruit.