Sadly, I see a lot of people being pretty dismissive of Arthur's chances this time around. I'm probably a little biased--he is my favorite video game character--but I do sincerely believe there's a great case for his inclusion.
Setting aside GnG's legacy as a Capcom franchise, Arthur's inclusion caused an absolute eruption at NYCC. I know Spencer is more popular than he used to be due to MvC3 hype, but when they were initially revealed the difference in crowd reaction is unmistakable:
https://www.youtube.com/watch?v=LL19fSP2kLk
And I don't link this to bash Spencer, just to show that people do genuinely want Arthur in the game and are excited to have him in the roster. At this panel Niitsuma also said he was heavily requested.
I think the lack of Arthur mains is more than likely due to how the Marvel 3v3 with assists format didn't do him any favors and, with that said, I think he deserves another shot in Infinite because this game looks like an environment in which Arthur would thrive.
No assists means people can't get in for free on Arthur and Arthur beats most other zoners 1:1 based on his projectile durability alone (assuming these values are consistent). I think, with no assists, Arthur would keep people out so much more easily. His strengths have only become greater. Even with projectile reflect, you likely wouldn't be able to do it on Torch, the Scythe and the Arrows (as the arrows would reflect above Arthur's head, most likely). Beyond that, since he throws multiple, I imagine if you reflected the first lance it would simply negate the other so Arthur wouldn't have fear of a projectile coming back his way. Overall I just don't see that as something hurting him too much.
His weaknesses: mobility, no dash? Infinity Stones, like Time for example. We now have Infinity Stones to give previously dash-less characters like Arthur a full blown teleport, among other crazy abilities. Plus being able to lock someone down with a torch and tag in while they're blocking the flames sounds like the kind of stuff MvCI is built around.
Plus people don't realize that, in Ultimate, Arthur had some of the most insanely impossible-to-block-mixups in the game. His instant over head was spammable and he could mix up his c.L and j.S at such a fast rate it's impossible to block on reaction. It's a complete guess. The pressure is crazy and what, if it happens now you're going to pushblock Arthur back to full screen in a game with no assists? Certainly not bad for him.
I know nobody really asked for a book about Arthur, and nobody necessarily implied he shouldn't be here or would be bad, but I think he's a pretty interesting topic this time around.