I got agree that some long ranged units should have ways to attack using melee. Using bows as melee doesn't sound realistic at all, so you could give them knives (The marauders already have them) or maybe using the same arrows as melee weapons.
This way long range units wouldn't become useless if they run out of ammo.
about hybrid units, yeah that sounds great, reminds me of the elves from the battle for middle earth:
They had an ability that allowed them to change between long range/melee units.
This could be interesting for dwarf axemen, they could have this 2 modes, 1 using axes as melee, and 2 using axes as long range weapons, throwing them.
then about light siege weapons
Humans use trebuchets, orcs catapults and elves ballistae/scorpions (Canons for dwarves)
So well these are already ingame, and there are more kinds of siege weapons you can buy in the bank. so I don't know which ones could be added, as there are already many.
Maybe for example a cart, propelled by horses, which would give high mobility and medium firepower.
I could see these in a undead race