I think Shiryu is going to have some hidden power, whether it's a suke suke awakening or some of the gorosei/holy knights magic. The magic stuff is where I'm leaning lately. His visual design has been compared to M. Bison, but he goes back earlier than that, to a very famous and influential Japanese story called "Teito Monogatari". He's based on the main villain, Yasunori Katō, who is a half-oni half-human sorcerer and descendent of the mythical Abe no Seimei, the legendary onmyoji and devil summoner.
Specifically, he's Kyūsaku Shimada's version from the 1988 film "Tokyo, the Last Megalopolis", with his iconic giant chin that makes him look like Antonio Inoki's brother.
Yasunori Kato isn't just a swordsman, so Shiryu using him as reference material was always kind of strange, because Kato is such an incredibly powerful and important magical figure. He's one of the most legendary magicians in the world. Back in Impel Down, I was wondering why Oda would use this character design for a regular swordsman, it seemed like a strange choice. But now that Oda has introduced the Gorosei, the Holy Knights, actual devil summoning with the Seal of Abe-no-Seimei (or the western pentagram, oddly enough they're the same thing used for similar purposes), I think Shiryu is heading for a significant buff of some kind.
Will he end up summoning shikigami? Maybe.
The "second strongest" spread also hints at some hidden power to me. Oda clarified that it wasn't the second in commands, but the second strongest person on the crew. What makes this regular ass swordsman who turns invisible the second strongest guy next to Blackbeard himself? He's hiding something.