Before the introduction of The Flash to the Arrow universe, Arrow was a really grounded TV show, and was able to introduce many characters in a believable way and use them effectively. A great example of this is the Suicide Squad which they made believable by using extreme assassins/mercenaries like Deadshot, Bronze Tiger, Shrapnel and Cupid instead of super powered villains. But now, with the Suicide Squad getting their own movie, they haven't been used as much and actually "killed" Deadshot so he can be only used for the films.
This might seem like seem like a negative, but actually it gives them the opportunity to create a new Suicide Squad and now that the Flash has been introduced for some time, we can see some meta-humans on the team. Now, determining who we will see on this new team is difficult, because if Deadshot is now a "movie only" character, what other Suicide Squad character are available? Nevertheless, I made a most likely new Suicide Squad list.
The obvious choice of who should lead this team is Slade Wilson (a.k.a. Deathstoke). Arguably the best villain we have had in the Arrow-verse hasn't been used properly since his defeat at the end of Season 2, and it would be great development for the character. It would also be an interesting dynamic to see Slade and Diggle working together.
In the mid-season crossover event that Arrow and the Flash did, they faced Captain Boomerang and he was a surprising threat for them. So much so that he was put in the same prison as Slade. In the crossover event, they actually hinted that Captain Boomerang was already used in a Suicide Squad that went wrong and was able to escape. His uniqueness with his weapon of choice makes him a very interesting and tactically smart character to use. I mean, seriously, who would think you would send a guy with boomerangs on a Suicide mission?
Even though Cupid isn't as liked as any other character, she still deserves to be on this list. She is still in the Suicide Squad because she fills the Harley Quinn role really well but also doesn't seem like a waste of space. She actually has some skill to back up her craziness.
Now we are getting into the meta-humans, everyone was surprised when King Shark actually showed up. Even though we don't know for sure, I don't believe King Shark has died (even if he did in Earth 2). The thick skin he has should have protected him. King Shark is a visually compelling character that would be very enjoyable to see in a Suicide Squad.
It's hard to know if we are actually going to see Caitlin become Killer Frost or if they even will do Killer Frost as an Earth 2 parallel dimension version. Ether way, Killer Frost is a great character for this team and her abilities and input in opinions would be a great dynamic to see.
Honestly, the character I've presented already are the main hitters. Other characters they use should be the ones that die to actually make it seem like a Suicide Squad. Characters Komodo, Clock King, The Huntress, Peekaboo and The Dodger are all contenders to die if they where a part of the Squad and would put some extra spice and variety to the Squad's teams.