It looks like Capcom left some extra images in their Gallery game files! Someone on Reddit has discovered some concept art of all the unreleased DLC characters for Marvel vs. Capcom Infinite. While you’re able to see Sigma and Black Widow in the game’s Story Mode, this art also includes first looks at Venom, Black Widow, and Winter Soldier. According to Shoryuken, these pieces are unlocked “in the Collection Mode after completing an Arcade Mode run with each character.”
You can see Venom above, who is rocking is classic comic book look.
Black Widow appears to have electric batons as weapons:

As for Winter Soldier, he will have a big arsenal available:

If you want to learn more about some of the DLC characters coming to Marvel vs. Capcom Infinite, we have the information right here: Capcom has revealed the four remaining DLC characters for Marvel vs. Capcom: Infinite?s first wave of downloadable content. Confirmed to be coming to the fighting game are Monster Hunter, Winter Soldier, Black Widow, and Venom. They?ll be sold individually, but are also part of the Deluxe Edition.
This raises the total amount of DLC characters to six, as Black Panther and Sigma were both revealed earlier this year. For those keeping count, the roster?s total fighters are now at 36, with the first 30 characters coming on the disc. While these characters are new additions, several of them do make appearances in Marvel vs. Capcom: Infinite?s story mode and wind up...