Bringing back Absalom’s DF and giving it to Shiryu is a pretty clear sign to me that Absalom was not using it to anywhere near it’s full potential. If it was a mediocre fruit, then why would Oda give it to Shiryu? Why give him this supposedly poor fruit when everyone was expecting diamond Shiryu?
I think we’ll be seeing Shiryu use it to levels that Absalom barely even dreamed of. An Awakened invisible fruit could be hax, not only would Shiryu be invisible, but he could make everything around him invisible. Imagine trying to fight a guy with Ray/Oden level physical ability, plus the ability to not only turn himself invisible, but turn everything around him invisible. He might even be able to turn Zoro invisible, so Zoro can’t even see his own body or tell where he’s standing. There’s a lot of things that can be done with Shiryu’s fruit that’ll force Zoro to use high levels of CoO as well as CoC and CoA.
And it’s not like Shiryu’s been idle for two years, him and the rest of the crew at the least have conquered most of Whitebeard’s old territory and beaten his crew (with Shiryu presumably beating either Jozu or Vista, depending on how strong one-armed Jozu is). And there’s something huge to come for the Blackbeard Pirates to boost Blackbeard’s bounty up past everyone else.
If Shiryu is Zoro’s final fight, you can be sure Oda will deliver on making him the biggest possible challenge. The thing with the Blackbeard Pirates that has to be remembered is, unlike most other villains, they haven’t hit their peak yet. Like the Strawhats, they are rising in strength as the series goes on.