There has been no official announcement of Hal Jordan. But there's too much to ignore at this point from BOTH Arrow and The Flash
Strong Easter Eggs
Constant mentions of Ferris Air cannot be a coincidence. For those who don't know, Ferris Air is the company where Hal Jordan worked for before his "disappearance."
WOW! Now if that doesn't scream "HAL JORDAN," I don't know what does. In Hal Jordan's original origin (Just a reminder if you've seen 2011 Green Lantern movie), Hal Jordan went missing during a test flight after the dying alien; Abin Sor's power ring chose Hal to be the next bearer of the Green Lantern ring
Hidden Easter Eggs
If you watched the Season 3 Finale of Arrow, you would've noticed that in the past, Oliver went on a boat that headed to Coast City. Coast City is Hal Jordan's birthplace in the comics. Hal Jordan is also a VERY good friend of Oliver in the comics as well; both being involved in many historical team-ups.
Super Hidden Easter Eggs
Many probably expected to learn at the end of Season 3 how Oliver learned to fly a plane. Well the CW has been very tight lipped on how and when he learned. But with Oliver heading to Coast City, where Hal Jordan resides, we may finally get our answer.
I don't find it coincidental that in the Season Finale of Arrow, we once again see Oliver flying a plane. To add to that, he seemed to be extremely knowledgable in doing so. Not only are Hal and Oliver great friends in the comics, but so are Hal and Barry. This would make for interesting character interactions. Can't wait for Season 4.