Two units I would love to see implemented even if just as single use ROR units or, as a concession, I would even accept them as campaign only(but with great sadness.) These are in order of seriousness of idea, in descending order.
1. Doom Diver that fires Orc Boyz instead. Jealous of the gobbos doing something that looks so fun. Probably a single artillery piece instead of a unit of them. The Queen Bess of hooliganism. Possibly same to shorter range, heavier damage, much funnier. The last point is the one that truly matters.
2. Black Orc cavalry. Heavily armored pig knights that charge stupidly hard. Could be super slow as a balance constraint (like 55 speed running, 75 on the charge) but overall just murderous, mountainous Orcs clad in half a steam tank of armor. Bonus points if the armor is literally made of salvaged pieces of steam tanks and other knights.
3. Shroom Tossers. Work like death globes except no direct impact damage, only provides a buff to friendly units and debuff to enemy units that go through it, possibly with a damage over time effect.
4. The Baconators. Smallish unit of monstrous cavalry, oversized boars with no rider that do trample damage and have an active stampede ability that makes them rampage, lowers melee defense but constantly try to keep cycle charging for 30s. Debuffed after.
5. Night goblin foreman. Exists entirely to provide gobbos buffs within his area. Basically a psuedo-necromancer but better combat abilities since he cant cast, but could have cool items.