yep. I was reading just this morning about the guest list they're using this season. It is very interesting,
but HIGHLY SPOILERISH.
In addition to Barry Allen, who sounds like he'll have a different origin for his alter ego, they're planning....
The Question (yay!)
Solomon Grundy -- now there's a bit of confusion on this one because supposedly Grundy will be in the Flash series, too.
The Clock King. Casting comments read: "the mind of a chess grandmaster, and can envision all the interlocking pieces of Starling City and adjust them in favor of his crimes."
I've also heard Talia, daughter of Ras Al Ghoul will play an important role. She's already been noted in the series at least once.
There have also been teasing clues posted which most people pretty much agree means:
Slade will finally, fully become Deathstroke
Red hood, which means either (A) Red Arrow finally dons his red hood, or (B) guest appearance by the Red Hood
A Suicide Squad storyline and Amanda Waller(maybe participating in Suicide Squad is done in exchange for a pardon for all the lives he took from season one?) -- Note, I've not see the Dec 4 episode as of this writing, but I think I've already seen a tease with Waller somewhere.
Then there are ones that are debated. These clues run as:
An earlier vigilante: some speculate this may be Vigilante himself (whom I only know from Justice League Unlimited animation). I read another theory that Wildcat is the earlier vigilante, which fits the Black Canary lore nicely. The most unlikely suggestion is Batman himself.
A better use for that clocktower. Some theorize this is just a comment on the the Clock King, whereas others note that with both Huntress and Black Canary now cannon to Arrow, the clocktower may become the Birds of Prey HQ.
A blank expression. Some have stated this as a reference to The Question, though he apparently is the answer to another clue. Another suggested possibility is Buddy Blank, aka OMAC. I don't know much about this character, but if they are reimagining him in this mutate-less world of theirs, a variation on the meaning of OMAC, One Man Army Corps, certainly sounds like a perfect soldier or mercenary.
"Rise" - no reasonable suggestion has been offered for this clue, near as I can tell