Well, I don't know if there's a "need" necessarily, but I don't think there's anyone playing the game that would say no if asked do they "want" more Hero classes! (aside from the 21 that voted XD)
I'm personally hoping to get a dedicated Rogue/Assassin class someday... that get the throwing blades as a starting weapon choice and not have to build them in the Forge! (personal gripe). Oh, and wrist-mounted mini-crossbows instead of lugging around the big ones on their back (so damn cool).
I know some folks are really keen on seeing the Ranger class tweaked to include some kind of optional animal companion that you can improve through a branch in their skill tree. Or something like the "Favored Enemy" mechanic from D&D, where you choose an enemy type (Animal, Fiend, Dragon, Giant, etc.) and do massive damage against those unit types.
I've seen a suggestion about a Druid class with a big focus on summoning and supporting animal, plant, and fey units, that can also shapeshift into an animal form during combat or something. That could neat.
Bard I could actually see working using mechanics already in the game, and could end up bring kinda' fun! Perhaps it has no direct damage attacks, but its skill tree could have a Chaos branch that inflicts debuffs like Misfortune, Demoralized, and Insanity on enemy units (like a Hero-version of the Skald), and an Order branch that buffs allies with increased Morale, Status Resistance, maybe refreshes their action points, etc.
An Engineer class would be also be pretty cool - it's actually a gripe of mine that I can't make a proper support hero for my Reaver factions that are all about supporting contructs and machines. It might be a tad too specific and situational though, but then again, it might work perfectly fine.