Spectre is one of DC’s strongest heroes canonically as a god-like entity capable of doing the impossible with ease, so his mere appearance would be quite significant.
Whether it will be the Oliver Queen or another character of another Earth played by Amell is up in the air.
In what may be the craziest rumor in terms of character appearances, Stephen Amell’s cryptic tweeting has some believing he’ll become another DC Character during “Crisis On Infinite Earths” and Arrow: Spectre.