I think HE are mostly in a fine spot, only they struggle a bit in the cavalry and infantry game. Perhaps artillery could be improved too. My suggestions:
"Dragon Princes - Halberds". Cost: 1500. 24 models, 234 HP per model, 36 MA, 32 MD, 50 CB. Since they do have a halberd, they should have AP and AL (28). Their models should visually represent the fact that they are a different variant of Dragon Princes, perhaps the Demigryph Knights models could be a good start. Also on these guys perhaps we won't have MP/MM just to not overshoot.
"Elven Militia": basically HE spearmen but cheaper. They should cost 300. Probably no shield, lower leadership and stats to compensate for the cost reduction.
"Great Elven Bolt Thrower": basically throws a larger bolt. Does more damage, flies in a straight line, more range and more damage (primarily AP). Penetrates ranks visibly more than a regular bolt thrower. Is much better at dealing with cav, killing 4-5 models per average hit compared to the normal 2-3 of the regular bolt thrower. Cost 800.
There's been a lot of posts arguing for more homogeneization. Let's not forget that some units lack some traits or are a bit weak by design and don't necessarily need buffs.