It's not speculation. The information is unreliable, but only because the patch is still in development, and subject to change. The list is not complete.
The source is someone who had access to the build, or access to people who played it at Canada Cup. Some of it was corroborated by several people, both on and offline, and it was too specific to be made up, so I put some of what I heard in a list.
Some stuff I'll add: everyone who has played this build of the game can't wait for it to be released. It's like a whole new game. Capcom is trying to change the current meta of the game where the most dominant tactic is prolonged setplay so that it will be all neutral. You land a hit into a combo into a mixup. But now, that mixup won't just mean putting them in extended blockstun while you set up degenerate hard to blockables.
Capcom does not seem to want certain characters to just be objectively significantly better than the rest. And for the first time, they're able to easily get the data they need to make these changes. Every time people find new tech, they upload it. And every time someone does something dirty on a stream, someone can clip it with the click of a button. Also, every match played has its replay automatically uploaded, including local matches (I think), so Capcom very likely has deep metrics (like global win/loss ratios) for every character in the game.
MvCI is shaping up to be the first Marvel game with a continuous development cycle, a DLC schedule, and the continued removal of bad shit like the web ball infinite and HSD glitch.

Marvel vs Capcom Infinite:
Season 2 as of the last update I heard from various sources compiled together will be Star-Lord, Ms. Marvel, Asura, Gill, and Lady, but the last character is one not everyone I've heard from has been able to agree on. There being multiple characters in development is likely to blame. The majority of them say Rashid while others have said Regina and one even said Apollo Justice.
Here's where things get really interesting. All the characters that I've heard that are in development (to various extents and from multiple sources, but not all saying the same things I might add, which is something strange in and of itself) include: Star-Lord, Ms. Marvel, Green Goblin, Wolverine, Cyclops, Psylocke, Magneto, Storm, Deadpool, Doctor Doom, Daredevil, Asura, Lady, Gill, Rashid, Captain Commando, Nina, Deis, Regina, Gene, Vergil, Akuma, and Apollo Justice.
I did notice the curious aspect of that being 11 Marvel characters and 12 Capcom. Which would line up with the current unevenness. But when you add it all up, that's 23 characters, not 24. Meaning there's still a Capcom character I don't know about, since adding these all up carries it all the way to Season 5 if it's all in fact true and all the sources are correct in different ways.
Four player 2 vs 2 mode is being added to the game, but I'm not sure when that is exactly going to be implemented.
I will say that despite the controversies with Marvel and the Fantastic Four in games lately, I haven't heard anything regarding scrapping Doom completely from the plans. While he isn't in the plans for next year from what I've been told, it's not conclusive as to his exclusion in the long term.
I guess it could be up to some people's interpretation what that means since he's not being added next year despite his popularity and all, but I wouldn't look too far into it just yet.
This next section for MvCI gets absolutely wild and so I am dedicating its own section to it. I have one source that's told me some stuff that I feel I have to share in the event it is true, but it is pretty out there. Do not take me as backing all of this either. I'm just putting it out there in the event this has ANY TRUTH TO IT later. They claim the game will get a free update sometime around the summer of 2018 that adds Deadpool, Wolverine, Daredevil, Akuma, Gene, and Vergil as free characters.
I will add that I was told by that same person that Regina was originally axed similarly to Ant-Man since both were actually planned to make it, but things came up that resulted in their discarding. Ant-Man in particular was due to technical issues like Frank West's on MvC3. I'm not totally sure what Regina's issue was, but I was told half jokingly that "it might be because of too many gun users." In spite of that, Ant-Man and Regina could pop up as DLC down the line since "some degree of work was done on her at an earlier point," and supposedly was picked back up on from what I've been told in Regina's case notably. And I was told she would/is going to be used to kind of crosspromote a Dino Crisis reboot. No one told me anything about Ant-Man since he was scrapped. But if the work is done and they wanted to approach him differently, I don't see why he couldn't make it back in. I've just heard nothing in regards to it yet. This same source was the only one who told me about Apollo Justice, which they claim is "unique" and utilizes Athena and Trucy in his moveset to some extents.
They go on to claim game is also going to get some kind of patch to fix the graphics and art style to improve the visuals. The UI is included in things being touched up with that.
When it comes to the roster, their goal is for MvCI to have "between 58 to 60 characters." Some things they're looking into adding include a Tournament Mode and a playable and balanced Ultron-Sigma patched in. A noteworthy thing about that is that they have some idea where you can have a boss character be your assist to compensate for no second character when playing as Ultron-Sigma.
Some more story-related content is of course being planned, with additional content being added to the current story mode, and a brand new one announced at SDCC 2018, with plans of that new story stuff being added sometime around late 2018 to Spring 2019 at the absolute latest. I'll end this section by throwing in something strange (if what they claim wasn't already) that I was told of by that same source, and it's so dumb that if it is rooted in fact, I am utterly stupefied by it. They say that early on, Capcom apparently wanted a Power Stone character. In particular Edward Falcon or Rouge, but they were specifically set on Falcon. Marvel ultimately turned him down because "they didn't want confusion with their Falcon." They said that for whatever reason, Capcom didn't fight them on it (which comes as no surprise there given how the roster turned out if this is true) or decide to go with Rouge instead as the Power Stone rep. That's all I got from that source, so now I can finally move on from this game's section.