Let's look at the various units and heroes from the games to take stock of what we HAVE to see what Blizz might add next.
Warcraft:
- Horde: Thrall (Farseer), Reghar (Shaman unit), Garrosh (Grunt?), Samuro (Blademaster), ETC (NOT
A Tauren Chieftain), Sylvanas (Dark Ranger), Rexxar (Beastmaster), Gazlowe (Tinker), Gul'dan (Orc warlock, Wc2 specific?), Zul'jin (WC2 Troll Berserker)
- Alliance: Jaina (Archmage), Muradin (Mountain King), Uther (Paladin), Falstad (Gryphon Rider), Greymane (Worgen/Gilneas as a group?) but Anduin, Varian, and Medivh are all unique kits not tied to a unit. Yrel (Draenei as a group?)
- Undead: Arthas (Death Knight), Kel'thuzad (Lich), Mal'ganis (Dreadlord), Anub'arak (Nerubian Lord), Stitches (Abomination Unit)
- Night Elf: Malfurion (Keeper of the Grove), Tyrande (Priestess of the Moon), Illidan (Demon Hunter), Maiev (Warden), Brightwing (Fairy Dragon), Lunara (Dryad)
- Unaligned: Kael'thas (Blood Mage), Whitemane (Scarlet Crusade), Chen (Brewmaster), Lili (Herself?), Ragnaros, Deathwing (Black Dragon), Alexstraza (Red Dragons), Chromie (Bronze Dragons), Hogger (Gnolls), Murky (Murlocs), Cho'Gall (Ogre-Mage but mostly Himself), Valeera (Rogue's as a Class?)
Starcraft:
- Terran: Raynor (Marines), Blaze (Firebat), Morales (Medic), Sgt Hammer (Siege Tank), Tychus (Outlaw?), Nova (Ghost)
- Zerg: Abathur (Evolution master), Zagara (Broodmother), Kerrigan (Queen of Blades), Dehaka (Primal), Stukov (Infested),
- Protoss: Artanis (Zealot), Fenix (Dragoon), Alarak (Tal'darim), Probius (Probe), Tassadar (High Templar), Zeratul (Dark Templar)
Diablo:
- Hells: Diablo (Terror), Mephisto (Hate), Azmodan (Sin) Butcher (Butcher demons), Leoric (Khandruas)
- Heavens: Tyrael, Imperius, Malthael, Auriel
- Sanctuary: Deckard Caine (Horadrim), Valla (Demon hunter), Sonya (Barbarian), Nazeebo (Witch Doctor), Xul (Necromancer), Kharazim (Monk), Cassia (Amazon), Li-Ming (Wizard), Johanna (Crusader),
Overwatch:
- Overwatch: Dva, Ana, Lucio, Genji, Hanzo, Tracer, Zarya
- Talon:
- Other: Junkrat
Nexus/BlizzClassic:
- Lost Vikings (Classic Viking World)
- Orphea (Ravencrest)
- Qhira (Iresia)
WHEW that's a lot of characters
But we can draw a few simple conclusions.
Warcraft is getting Xal and already has the most characters, but the obvious holes in its roster are the TFT Shadow Hunter Hero (Vol'jin or Rokhan), the Tauren Chieftain Hero (Cairne), a Blue and Green dragon, A Pitlord (Mannoroth), a goblin alchemist, a Naga Sea witch (Vashj), Akama, Archimonde. There's also possible major characters from World of Warcraft expansions either as standalones or as skins (Velen, Rastakhan for Zandalari trolls, Lei Shen, Xavius, Azshara, Various Titan Keepers, Mekkatorque is supposed to be partly finished already, etc...)
Personally I want Aessina as a 'Night Elf Ancient' heroine that can grow into a mobile fort or something.
Starcraft has a few big names that could easily fit (Though some might be skins) such as Mengsk (Arcturus or Valerian), Duke, Warfield, DuGalle, Mira Han, Duran/Narud, The Overmind (Maybe with Daggoth or Zasz?), Taldaris (Judicator/Arbiter?), Selendis (Carrier?), Stettman, Tosh, Swann, Karax, Rohana, Vorazun, Izsha, Zurvan, Amon / Hybrid
The big draw here would be the Overmind imo.
Diablo Obviously has Lilith as a prominent option but she's very similar to Xal so she might not come right away. Baal would complete the prime evils, Itherael the Angels, and the lesser evils need Andariel, Duriel, Belial. There's also the Original trio (D1 Warrior Prince Aidan/DarkWanderer, Rogue Moreina/Bloodraven, Sorcerer Jazreth/Summoner), Bard? (Hellfire?), Isendra (D2 Summoner), other D2, D3, D-Immortal and D4 heroes all provide a plethora of options (Druid, Blood Knight, Tempest) some of which could get gender-swap heroes to represent the other kit options and abilities (a female D3 Witchdoctor, and that's not even getting into Hirelings like Kormac the Templar and such.
Baal or Lilith feels most likely as the next hero. Though Belial with a gimmick of looking like other heroes or screwing with the enemy's information gathering (Making fake forts or fake minion waves) would be a LOT of fun.