Let's talk about the Hatred mechanic that's been missing for far too long.
In Total War terms this equates to a roughly 50-60% Melee Attack Bonus for the first round of combat (20ish seconds). How exactly would this skill function and when?
Active if: In Combat
Recharges if: Not In Combat
Effect: +50-60%% melee attack
Duration: 20 seconds
Recharge: 40 seconds
It could also use the charge mechanic and have the bonus fade over time, representing the initial fury burning itself out.
Some common examples of units with Hatred:
Basically the entire Dwarfs roster has the hatred special rule, but only against Greenskins & Skaven most of the time. For the rest they had to roll to see if they hate you. Since Dwarfs don't use magic and don't need to roll for Winds of Magic, they could instead roll for grudge at the beginning of each battle to determine whether the army has Hatred active for the battle (assuming they're not fighting GS/Sk where it's 100%). Higher level lords and LL could have skills that give a bonus to increase the chance of having a Grudge against that particular army. A basic chance might only be 33%, but a higher level lord has read through the Book of Grudges and can find a reason to be angry at just about anybody.
The other really hateful faction is the Dark Elves where every intelligent model has Hatred, but only of High Elves. They also have a unique special mechanic called Eternal Hatred available to Black Guards, Malekith, Malus and Kouran where they always hate everyone, in every round of combat, forever. I think that might be a tad overkill balance wise in Total War, so instead I would recommend just letting it recharge while in combat instead of needing to be out of combat for 40 seconds to be able to unleash their hatred again like other units.
These are far from the only units in Warhammer that benefit from the Hatred special rule. Shockingly, Grimgor gets Hatred (Everybody!), and Night Goblins really hate the stunties. Warrior Priests can get their flock whipped into a frenzy and give them Hatred. In game terms WPs have Hatred and grant it in an small aura.
There are many more examples of this important, thematic and unrepresented special ability, but you get the idea. This is a mechanic whose time has come.