Now before I get started, this is more dedicated to obscure heroes and villains, so hero-wise you will not see the likes of Silver Surfer, Namor, Jubilee, or Eddie Brock (Venom/Anti-Venom/Toxin) they all have pretty vocal advocates for their addition, and will likely be added in time, as do some villains like Carnage, Annihilus, and Galactus.
This is more about obscure heroes and villains, I am also ignoring pre-existing villains as LB heroes on my heroes list, many of them being so obvious they will probably happen regardless of what any of us say (Mystique, Avalanche, Blob, Toad, Sandman, Winter Soldier, Venom (Mac Gargan), Green Goblin, Bullseye come to mind)
So here are the ones I hope to see with all that in mind:
10: Darkhawk: I think as a New Warriors/Avengers member he would be great, he also has recently had some fame as being one of the few survivors of the limited series Avengers Arena.
9: Prowler: An obscure Spider-Man ally/enemy who has been part of Spidey's team "The Outlaws" alongside Sandman, Puma, and Rocket Racer, he is also a much appreciated addition to the roster of black super-heroes.
8: Silver Sable: If I have to explain why this gun-toting, sword-slashing, mercenary babe from Symkaria needs to be added, then you just suffered some major head trauma.
7: Dark Angel: Shevaun Haldane is probably one of the more memorable Marvel UK heroes, she led the Dark Guard, has a grudge with Mephisto for killing her father, and recently was seen fighting Dracula alongside the likes of Blade and MI6. Her costume itself is a black hole.
6: Death's Head II: Minion is freaking awesome, as a member of the Dark Guard/MI6 he has proven to be one of Earth's most powerful heroes, also his power to absorb people's minds and skill with his "cranial syphon" would make him a very cool generalist.
5: Motormouth: Would really love getting to recruit this fowl mouthed teleporter!
4: Death's Head: Marvel's resident time/reality hopping Mechanoid Freelance Peacekeeping Agent (don't call him a robot bounty hunter) would be an excellant addition to the roster, yes?
3: X-51/Machine Man/Mr. Machine: One of the few Avengers members I hardly ever see mentioned, X-51 has been a member of the Avengers, and an ally of the X-Men, his technopathy could be used in some fun and interesting ways.
2: Werewolf by Night: Show of hands... Who would not like to unleash a deadly bloodthirsty werewolf on their enemies? Jack would be an awesome addition to bleed teams.
1: Sleepwalker: Just so many possibilities for him, while he mainly just melee attacks with his fists, and occasionally uses a weapon called the "mindrake" he has a very interesting power: the ability to alter reality simply by looking at something, his "Warp Gaze" he can warp the very fabric of reality at a glance, he does not use this on living people, not because he can't, but because he feels it would be wrong, imagine if he was not bound by that personal rule...
10: Vermin: an insane, cannibalistic rat-man, Captain America/Spider-Man villain, also a Baron Zemo henchman at one point.
9: Jackal: Punisher's initial employer, an enemy of Spider-Man, and the creator of such spidey clones as Scarlet Spider, Kaine, and Spidercide.
8: The Carnage Family: If Carnage is ever put in the game, I hope his gang of killers come with him, Shriek, Doppelganger, Demogoblin, and Carrion.
7: Acolytes: With the Brotherhood gradually being phased into heroes, a new mutant villain team should emerge, my vote is for the Acolytes, Fabien Cortez, Delgado, Seamus Meloncamp, Senyaka, Spore, Exodus and the rest would more then fill the void of a heroic Brotherhood. PLUS Acolytes Colossus alt!
6: Charnel: The big bad of the Death's Head II comics, he slaughtered virtually every hero and villain of his reality.
5: MyS-Tech: Sorcerors/Sorcerouses who sold their souls to Mephisto millenia ago... What are they doing now? While running a museum of "Paegan Antiquities" and managing a multi-national corporation... Yeah evil rich immortals who sold their souls to the devil for power... Oh yeah, and they kinda want to destroy the multi-verse as an endgame... which would fit in well with Incursions and the CO8.
4: High Evolutionary/Knights of Wundagore/Ani-Men: Would be great spec ops enemies if we ever visit the Savage Land.
3: Slayback: Deadpool needs a villain, this guy should be that villain.
2: In-Betweener: If we EVER get Silver Surfer, a good cosmic enemy who has been beaten but is uber powerful would be nice to have, I think In-Betweener fits that build.
1: Joseph: Magneto's clone, both villain and lockbox villain material at the same time.