The Marvel side is just fine, just bring every marvel character back from mvc2 and add deadpool and thor.

The Capcom side on the other hand needs a major overhaul because there are WAY too many street fighter characters, its no wonder marvel was dominating the whole game.

Try to make the capcom roster as diverse as possible

Capcom Roster:

Ryu from Street fighter

Chun li from Street Fighter 2

Alex from Street Fighter 3 (bring it on juggarnaut)

Morrigan from Darkstalkers

Batsu from Rival Schools

Hayato from Star Gladiator

Leo from Warzard

Megaman X from Megaman X

Dante from Devil May Cry

Samonasuke from Onimusha (I will have your head silver samuari)

Jill from Resident Evil

Regina from Dino Crisis

Date Masamune from Sengoku Basara

Ratholos from Monster Hunter

Phoenix Wright from Ace Attorney

Simone from Cannon Spike (eat this psylocke)

Viewtful Joe from Viewtful Joe

Ammy from Okami

Ryu(B) from Breath of Fire Dragon's Quarter

Will from Dark Void (some one needs to give Cable a taste of his own medicine, you too iron man)

PTX-40A from Lost Planet (sentinel got competition, and yes ptx-40a can team up with other characters)

G.Kaiser from Tech Romancer (sentinel got more competition)

Arthur from Ghosts N Goblins

Strider Hiryu from Strider

Bionic Commando from Bionic Commando(2009) 

Captain Commando from Captain Commando

Jin from Cyberbots

Hagger from Final Fight (bring it on hulk)

Saki from Quiz Nanairo Dreams (some one needs to give cable a taste of his own medicine)

Unknown Solider from Forgotten Worlds (some one needs to give cable a taste of his own medicine)

 

Oh and, Apoclypse, Abyss, Onslaught, and Yami are made into playable characters.

 

well certainly  would like to multiple versions of the same character like they did wolverine.  Like when you select a character you can choose which series to use the character from.  Like the megaman's would be the biggest example but same goes for dante, and spider-man etc.  It would be like selecting a different costume except the character plays different, you should be able to select from a drop down menu or something of the sort.  Maybe, when you highlight the character it displays the different selectable versions around it in a sub-select menu.  This could expand the roster without adding anyone "new."  But I see this working really well if Dante was added, because his game would alter his demon form and weapons.  The same applies for spider-man in his symbiote and costume and stark armor. But i think this mechanic would add a lot of depth to gameplay.

Here is my list anyways:

 

Marvel:

Spider-Man { Has To Be There ... Grr }

Hulk

Iron Man

Thor

Wolverine

Storm

Mr.Fantastic

The Thing

Human Torch

Invisible Woman

Jean Grey

Psylocke

Emma Frost

Cyclops

Wasp

Giant-Man

Black Panther

Blade

Ghost Rider

Silver Surfer

Nova

Cable

Capcom Side :

B.B.Hood

Felicia

Anakaris

Morrigan

Jedah

Dante

Viewful Joe

Phoenix Wright

Ryu

Chun-Li

Guile

Cammy

Megaman

Roll

Zangief

Strider

Jin

Sexy Silvia

Jill Valentine

Leon.S.Kennedy

M.Bison

Captain Blue

Servbot

If they do a third one, I would like them to use the character select from marvel vs capcom 1, that was the best selection theme out of all versus series. Also Capcom, do not be afraid to bring out your heavy hitters, I wouldnt mind being cheap with giant robots like G.Kaiser or Blodia, its about time some one taught Sentinel a lesson.

I am a big believer in the Vs. series going the 'bigger and better' route. I know more characters would make balancing issues an even bigger mess, but let us be honest... We don't play MvC2 for the balanced gameplay. We play it because it is huge, flashy, fast, and above all fun. I want ALL of MvC2 to make a comeback.

On the Marvel side I would definitely add a lot of new ones, because so many characters have risen to the forefront of the Marvel universe in recent years that would just be fantastic.

Nightcrawler.

Thor.

Deadpool.

Black Panther.

Hank Pym (Giant Man, Ant Man, Yellowjacket)

Sentry.

Super Skrull.

Black Bolt.

Scarlet Witch.

Green Goblin.

Penance.

Daredevil.

 

 

On the Capcom side it is even easier to recommend new characters. They have created numerous fantastic new franchises in the past decade.

 

Zero (Mega Man X)

Mega Man X (Mega Man X)

Phoenix Wright (Ace Attorney)

Miles Edgeworth (Ace Attorney)

Dante (Devil May Cry)

Samanosuke (Onimusha)

Amaterasu (Okami)

Frank West (Dead Rising)

Arthur (Ghosts n Goblins)

C. Viper (Street Fighter IV)

Bionic Commando (Bionic Commando)

Leon Kennedy or Chris Redfield (Resident Evil)

Albert Wesker (Resident Evil 5 version)

Joe (Viewtiful Joe)

Gene (Godhand)

 

Also I think they could go a step further with alterate costumes instead of just swapping a color palette. Here is hoping!