I think a big unit for the humans would ruin their aesthetic. The orcs look and work great with their single large units, and the elf-treant connection works great too. I mean, it's great to branch off ideas, but in my opinion any kind of giant war machine, creature, or anything else just wouldn't make them feel human anymore.
What would be awesome is if you had different cultures of men, like say eastern-style desert dwellers. THEN you could add in elephants, chariots, and even half-troll or half-orc units and it would look absolutely fantastic. But since the humans are pretty European, I'd say it'd be best to keep them that way.