Let's be clear about this: Ultimate Alliance 3 has quite the roster. There are thirty-six playable characters in the game right now, with quite a few more in the plans for future DLC/updates. Some of the future characters are confirmed, and others are being kept under wraps. That being said, there are plenty more characters that would be a blast to play as in a possible Ultimate Alliance 4. While it's possible that some of these characters may be added in future DLC's for Ultimate Alliance 3, some of them most certainly will not, but could appear in a sequel.
(Keep in mind, this list is going to contain minor spoilers for Ultimate Alliance 3)
10 X-23
While her abilities would certainly be similar to Wolverine, there are about a dozen spider-people who all have their own awesome abilities Ultimate Alliance 3, so we're pretty sure they could make this work. X-23 (Laura in Logan) could be an absolute blast to play as. In the comics she's typically depicted as a bit older than she was in the movie, even donning the costume and name Wolverine for a while. Her regenerative abilities alone would make her a valuable asset on the team, and a combo of her and the original Wolverine would be hilarious fun.
9 Cable
Another character from the X-universe side of things, Cable is a character they could do quite a lot of different things with. He could have a ton of different gadgets/weapons, giving him a bunch of different synergies to work with, which could make him a good fit for a lot of different team compositions.
Cable is a super-popular character in the X-Men fandom, and a lot of people would be happy to see him included in a sequel.
8 Jessica Jones
Jessica Jones does appear in Ultimate Alliance 3, though she is not a playable character. (At least not yet) While she doesn't typically fit into the usual superhero bracket, she definitely is on the side of good.
Star of her own show that was recently canceled by Netflix (grrr), Jessica Jones has super-strength and a sarcastic personality. There are quite a few heroes with super-strength already in Ultimate Alliance, but at the same time, there's a lot of different things you can do with super-strength, too.
7 Ant-Man
Ant-Man makes an appearance in Ultimate Alliance 3, but isn't a playable character. (Wasp is, though, and she's awesome!)
Having the option to pair both Ant-Man and Wasp together could be a great time, with both characters shrinking (or growing, as another power possibility for Ant-Man) throughout the fight which could make for a really unique and interesting playstyle.
6 Colleen Wing
The crime-fighting partner of Misty Knight (who would be another awesome addition, by the way) and, occasionally, Iron Fist, Colleen Wing is a Japanese martial artist who avenged the death of her grandfather. She typically uses a sword, though she's well-versed in many different forms of martial arts. There's no such thing as too many characters with martial arts in our opinions, so the inclusion of Colleen Wing in the next Ultimate Alliance would certainly be a welcome one.
5 War Machine
A good friend of Iron Man, War Machine has his own unique armor he uses that does actually differ from Iron Man's. War Machine's armor is typically a bit less maneuverable than his buddy's, and with his heavy emphasis on his shoulder-mounted weaponry, it's no wonder.
War Machine is always packing quite the punch with a plethora of different hard-hitting weaponry that would both make him wildly different to play than Iron Man, and could also make him a good teammate in just about any team composition.
4 Jean Grey
Jean Grey is a no-brainer, here. She could be portrayed as either Jean or Phoenix (or Dark Phoenix, if they want to go there), and her powers could allow for quite a lot of different options. Her telepathic abilities could lend themselves towards a pretty-nice crowd control character like Spider-Gwen. They could also give her some nice team-oriented powers, like giving her teammates telepathic shields, or something like that. There's a lot of different territory they could explore with Jean, and I don't think anybody would complain about seeing more X-people added to the roster.
3 Iceman
Another character that could be added in the X-Men DLC that will include Cyclops and Colossus, Iceman would be another fan-favorite character to add. He would obviously have some pretty specific synergy abilities that could pair well with other characters using elemental powers, but he's another character who could have some interesting crowd-control capabilities if they wanted to go that direction, too!
He could conceivably cover the ground in ice, making it difficult for enemies to maneuver, or encase them in ice entirely, preventing them from doing much of anything.
2 Valkyrie
A standout fan-favorite character ever since Thor: Ragnarok, Valkyrie could give Ultimate Alliance some more Asgardian flavor. Valkyrie could bring a wide variety of weaponry along, allowing her abilities to make use of a pretty diverse arsenal. First appearing way back in 1970, Valkyrie has been in a ton of different Asgardian stories since. She's also been known to work with Misty Knight, so her inclusion with Collen Wing and Misty Knight could plausibly happen. And if we happen to go to an Asgardian realm in Ultimate Alliance 4, we'd say Valkyrie is probably a sure bet.
1 Quicksilver
This is a character that could, if done right, be an absolute blast to play. Quicksilver is the speedster brother of Scarlet Witch and the son of Magneto. He's had a heavy role in quite a few different comics in the past, though his inclusion in the MCU was pretty limited. (He's gotten some cool stuff to do in the X-Men movies, though!) Quicksilver's speed powers could be stupid fun to play, possibly allowing him to take care of multiple enemies at pretty much the same time, or put a ton of damage on a single enemy in the blink of an eye.