I propose this theory: We go back to the original timeline. I'm going to be calling the original timeline the "white circle timeline" and the one that we have watched this season the "red circle timeline."
I'm just going to go ahead and say that the massive black hole exists because it is trying to erase a timeline that doesn't exist. With no Eddie, there should be no Reverse Flash, which means he wasn't there to change the past. This means both of Barry's parents are alive and we will see the white circle Flash.
Let's talk about Tom Cavanagh for a second. We know he'll be back. In the white circle timeline, we know Harrison and Tess Wells built the particle accelerator in the year 2020, and that's when Barry got his powers. If Reverse Flash didn't come back in time and kill the two, they should still be alive then, right? I think we'll see these two mentor Barry in season 2, just like we heard they did in the white circle timeline.
Now moving on to Reverse Flash. He's gone, now. Or is he? He is the Reverse Flash, it wouldn't make sense to never see one of the greatest Flash villains ever again on the show. But how can Eobard Thawne exist? Easy, his ancestor in the original timeline was Malcolm Thawne, aka Barry's twin Brother, aka Cobalt Blue. At least, this is just a theory... My friends and I just wanted to see Cobalt Blue at some point. I guess this is less theory, and more @i want this."
Anyways, we can resolve a lot with this timeline. Cisco can have become The Vibe by now. Caitlin will have become Killer Frost. Maybe in the white circle timeline, Ronnie isn't Firestorm. Dr. Stein and Jason Rusch are. Listen, if it's established that Ronnie died due to the whole Firestorm project, then it would make sense for Caitlin to hate Firestorm and become a villain. In addition, as we saw in the Legends of Tomorrow trailer, Dr. Stein says he is only one half of Firestorm. Every one is assuming Ronnie is dead, and that is the reason. I want to say, yes that is true, but I think there's more to it. For the trailer, they didn't want us to see Franz Drameh's character, "Jay Jackson." Jay Jackson may actually be Jason Rusch, who is being recasted (we've seen this with Sara Lance before...). In-universe, the explanation may just be that Jason was not on-time to this big meeting between "legends." Out-of-universe, again, they just didn't want to spoil the new timeline. Another character in this show who may have been changed: Hawkgirl. At he end of The Flash, she literally did nothing as everything was ending. Just stood and watched... Didn't try to fly at all... What if she didn't have her wings at this point, wasn't aware of her past live yet? In the white circle timeline, with the events of all the shows taking place in 2020 instead of 2014 would make that easier.
In the Arrow finale, we just assumed The Atom did not die, and instead shrunk. I say, he did die. In the white circle timeline, he doesn't die, and instead become The Atom we know he will be. Speaking of Arrow, I think the white circle should affect their show, too. The season 3 finale felt very much like a series finale, which was intentional, and with all this talk of a light-hearted season 4... What if the show came back as Green Arrow? In the red circle timeline, with the whole "Roy Harper is The Arrow but he's dead now" thing, The Arrow can't exist. And calling himself the Green Arrow won't change that. We were promised a journey to see Ollie become the Green Arrow. We got to see an Oliver who became a hero, and hen retired happily. We got to see Ollie happy, which is one part of the way to him becoming Green Arrow, but not all the way. Maybe we needed to be following a different Oliver? Maybe Ra's was defeated early enough so that Ollie's identity wasn't revealed to Captain Lance. I also say, what if since the moment Oliver returned from Lian Yu, he was actually more wise-cracking Oliver than brooding Oliver; what if he was never The Vigilante, never The Hood, never The Arrow, but only Green Arrow?
Anyways, there probably won't be any other big changes there: Malcolm is likely still Ra's, Thea is probably Speedy (or maybe even Arrowette or Artemis), Roy is probably off on his own for whatever reason, Diggle will be more open to putting on a mask in this timeline, etc. the only other big difference, is Sara. It doesn't make sense for a decaying corpse to just be completely okay after a dip in the Lazarus Pit. It's been speculated Rip Hunter and time travel was involved. I propose, in the white circle timeline, right after Sara died, Nyssa immediately resurrected her. I think Nyssa and Sara may both join Team Arrow as well.
In the LoT trailer, we see Ollie and Barry side-by-side, or well, more specifically, The Arrow and The Flash (red circle). Just like before, I think since the scenes was shot specifically for the trailer and not the show (although they can reuse scenes), that was just placeholders. They likely didn't want to spoil the events of Green Arrow and The Flash's second season by debuting the new costumes. Those two could possibly end up being Green Arrow and The Flash (white circle), instead.
Also, the white circle timeline can make it easier to connect to the Titans (or Blackbirds?), Supergirl, and even Constantine.
That's all I got right now, keep in mind this is all just a theory I came up with to make things easier for me to explain to myself and my friends what may possibly happen in season 4 and season 2 of Arrow and The Flash, respectively. And Legends of Tomorrow, can't forget that.