It is true that Capcom originally wanted 58 characters. The extra 8 characters were never revealed. I like the game how it is too, but I wish they would've put in the extra 8 characters and put a few more characters as dlc. There were a ton of fan demanded characters like Gambit, Venom, MegaMan, etc.
I read an interview that Niitsuma (personally) wanted 2 characters added: Rogue from Marvel and Gene from Capcom.
I doubt very seriously if we will get anymore characters or even an update for this game. I like the game, but it had SOOO much potential to be even better.
I guess we will have to hope for MvC4 or a new Marvel or Capcom game with the characters we want.
Even if they added all the characters they wanted, I doubt Venom and Gambit would have been included. As I've said before, Marvel has the final say on their characters as it's their license. The majority of the Marvel characters were put in for promotional reasons. The Venom most people want no longer exists. Brock hasn't been Venom for years And if they put in Flash Thompson Venom instead, people would be complaining.
And up until just very recently, Gambit had a pretty good lack of exposure in the comics. Around the time both versions of MVC3 were in development/came out, all he was doing was just playing nanny to X-23 in her solo. And doesn't seem to be as popular as he was ten years ago. To give you an idea, based on Comic Book Resources top 50 most popular Marvel characters poll last year, he ranked only #34, barely ahead of Nova and X-23, two characters with a fraction of the exposure as he did, and he didn't even get top 10 among X-men characters.
This makes me certain that most of his popularity stems heavily from the massive exposure he got in the 90s, namely the cartoon series + all the games that reaped off that show's success (though, he wasn't in X-men Children Of The Atom, strangely enough). His exposure since then has been getting less and less. To illustrate? In X-men Evolution and Wolverine & The X-men, he only made guest appearances. The 90s X-men was the only cartoon he was a main character in. And after the X-men movie franchise began, it took almost ten years for him to finally appear. And that wasn't even an actual X-men movie.
If I'm correct, the only Marvel characters that are in that Marvel didn't want and Capcom had to pull some strings to get in are Shuma-Gorath and Sentinel. Capcom seems to like Shuma for some reason. Apparently it's because he was the first non-humanoid character in their Marvel FG. And Marvel only agreed to have him put in if he was DLC only. And Sentinel got in mostly due to his MVC2 legacy.
Interesting thing about Sentinel is that he was probably the one MVC2 Marvel character that had the lowest chances of returning.(OK, maybe not lower than Bone Claw Wolverine, but everyone else seemed to have bigger chances) When MVC3 was announced, I don't recall once ever seeing him on someone's wishlist. He was one of the four god tier characters in MVC2 and at least 80% of the players had him on their team. It was mentioned that Marvel told Capcom that they could use either Juggernaut or Sentinel. But not both. Due to Sentinel's popularity at tournaments, Capcom most likely went with him. However most casual game players don't care for Sentinel. So if the roster were completely fan-driven Sentinel never would have made it in. I admit, that I wasn't too pleased with his inclusion and would have preferred other characters, but I got over it.