On a podcast called Tomorrow will be Televised, Johns said before the first episode released we're going to see villains/characters that haven't been advertised. Becky is surely one because she is Hazard in Infinity Inc comics. Helix was teased as something that wanted to figure out Todd's powers, so I can see her and other minor Infinity Inc/JSA characters like Chroma being connected to Helix for the same reason (if they aren't just staff instead). And last episode Courtney said she wants to find Becky which could officially weave in Helix into the storyline.
As for theories for the killer, most of the ISA and Shiv definitely aren't it (at least under their own volition..) because they are being thrown around as suspects too early in the season. Makes for good drama/distrust to drag out for like 6 episodes though.
Syl coming back wrong because of Helix/Bones and being the killer seems to be a pretty common theory from what I've seen. And I get why people would think that, he's been acting fairly aggressively and unhinged, though I think his behavior could be explained away pretty easily (survivor's guilt, rightfully wondering why his teammates killers are running around). But trust seems to be a big theme of the season, and Pat (and Courtney by extension) trust him a lot and have no reason to suspect him yet.
Todd being the killer via manipulation or forced to by Bones is a common theory too for similar reasons. They trust Jennie and would probably trust him by extension, and Courtney trusting him only for him to do something horrible would be in contrast against Courtney not originally trusting Jennie despite her being nice. Also this is Johns and Robinson we're talking about, they're the ones who wrote Todd's villain storyline in the comics in the first place, surely they can't resist doing it here either.
I see Ultra Humanite a surprising amount too, probably as a leftover from when Syl coming back was first teased and also just plain wishful thinking lol. Though "the killer has many names" DOES make sense for Ultra Humanite when everybody who's anybody on the show has a minimum of two names...