I think that once Ollie experiences whatever he needs to in order to stay on as Green Arrow as protector of city (I hope they use the name Green Arrow in a serious context in the last episode) Felicity will take up the mantle of Oracle, Diggle will be Arsenal, and Roy will be Red Arrow.
Having Diggle emerge as Arsenal would be interesting and sets up sort of a pre-manifest destiny Roy taking up that mantle maybe perhaps later. It's certainly not too different with how Black Canary's character has changed over the years (which this series has yet to pay homage to, but still in theory could).
I really don't know why Felicity is being pegged as Oracle. Oracle is such a very specific person and you're (as in everyone mentioning this idea) is talking about a very popular character from an even more popular franchise.
No, full stop. Felicity WILL NOT become Oracle. Can you imagine the nerd rage that would occur if TPTB took that from Barbara Gordon? There's a very, very big difference between making Diggle into Arsonal; a moniker that already exists in the franchise for a character that has the possibility of still assuming it later; or dividing Black Canary into both a mother and a daughter of a character who have the same name and fight crime under the same moniker. It's completely different ripping a moniker from another franchise for a character that is barely a blip on the radar and installing it on her. Before even considering how wrong that is, think about the potential legal issues they'd run into. I'm sure the Fox estate would sue for all of Arrow's Microsoft advertising dollars >.<
Let's put this in perspective where Felicity is concerned: Before this series occurred, Felicity Smoack was the owner of a struggling software company that on several occasions was the victim of super hero shenanigans. It inadvertently took collateral damage to the point that Felicity was a huge supporter of programs like Cadmus. I'm not sure if we'll see any nods to this history with this character in this series, but it's something to keep in mind.
I think it is true she'll need to take on a moniker at some point after this incident of being discovered but I suspect they'll either make something up or they'll use a name of a character that is barely known and who has no known alias yet, such as Software. Heck, considering they've gone 22 episodes so far calling Green Arrow nothing more then "The Hood" I'd expect Felicity could go 22 episodes next season being known as nothing more then "The Hacker".
I really don't think they are going to make most of the characters into heroes. At most Roy and at some point Laurel. Even then I suspect for Laurel it would be a limited thing. She'll go undercover at a nightclub or something while working on a legal case and just happen to use a blonde wig and something resembling the canary outfit as her disguise.
It's so very hard to say at this point. I'd venture to say if anything, masked heroes will be added as seasons go on. This season has obviously been mostly a setup of the characters. I think over the course of next season we'll see Roy and Laurel struggling with the annals of crime fighting (or rather, ethics there in) while Diggle and Felicity lock in some sort of working moniker. I suspect early to mid season Roy will take up the mantle as Speedy or Red Arrow and mid to late season Laurel (or maybe her sister) will take up the mantel as Black Canary. I'd actually be really surprised if we don't learn Laurel's mother didn't doan the Black Canary mantel for a period of time (possibly slugging it out with Merlyn). I'd also be surprised if we don't meet Wild Cat next season and he doesn't have some major part in the happenings.