Leadership: we should definitely get a Gamling or Grimbold hero, preferably Gamling with some effect to inspire Rohirrim with his banner.
Tactics: I would also LOVE to see a new Noldor in Tactics, but I think the game needs Gwahir for an eagle hero finally.
Lore: I think Rossiel really needs a partner for victory display, which could maybe be an Orophin with attack power that increases against enemies in victory display who share a trait.
Spirit: I reckon a Gondor or Outlands hero like Angbor the Fearless with Lamedon allies OR now that we have Derufin spoiled, maybe Duinhir his dad with some Blackroot Vale archers would be nice. (Not to mention Forlong the Fat as a hero would make a great defender).
Neutral: I think Saruman is definitely one that would be a lot of fun to play, with some effect to switch spheres each turn as he has many colours.
I would also really want to see Aeglos, Gil-Galad's spear as an artefact weapon for tactics... could be a guarded attachment for +2 attack and something for Noldor synergy.