So I'm sure that there's a huge number of ideas regarding this already but I thought I'd throw in my two cents anyway.
I think, and correct me if I'm wrong here - the point of fightiness, the point of WAAAGH!s is to encourage a kind of snow-bally, aggressive, can't stop: won't stop, sort of play on the campaign map. It's a cool concept that ties in well with the greenskins lore - it's just that WAAAGH!s are dumb, tend to do not a whole lot other than lag behind the player controlled army until they get wiped out by being stupid and even very late game tend to not have very good units (wurzag waagh!s being an exception).
I like fightiness as a principal - go out there, fight, win, snowball to hell and back until you hit a brick wall and have to start from scratch, and I think that little green (sometimes red) bar next to every lord's portrait could (maybe even should) be the backbone of the entire greenskins' rework.
Firstly: ditch WAAAGH!s ... kind of I'll get back to that. No more random ai controlled dumb army that lags a turn behind you when you hit 80/17 - it's cool at first but honestly they suck.
Secondly make greenskin unit replenishment tied exclusively to fightiness - no bonuses for being garrisoned/encamped - no requirement to be in friendly territory. Terrain penalties should still apply of course, and with wurzags settlements buildings they should offer a bonus to how much replenishment each point of fightiness gives (or another bonus of some kind to savage orcs). Oh and the replenishment can go negative and cause attrition of course.
Thirdly, make fightiness give the army a combat stats buff - the boyz are pumped up, on a roll and ready to smash skulls - they gonna bash better cause they're in the right mood. Give regular greenskins small offensive bonuses, reload speed, missile accuracy, weapon damage and melee attack. Give the bloody hands movement speed and charge bonus.
Fourth - back to waaagh!s Allow the player to with research (unlocked by appropriate buildings) spend fightiness to recruit units anywhere and at 100% fightiness to recruit a lord on the spot.
Fifth - make the gains/losses of fightiness more dynamic - minus 1 to animosity per turn doesn't exactly make going out and fighting feel like a massive priority when I've no fear of facing attrition or losing my WAAGH! for the next 20 turn. Especially when there's no real benefits for having high fightiness unless you tip it past the magical number. Making the losses of fightiness be percentage based would make pushing for max fightiness to expand your military while you push a genuine challenge. And making wins on concurrent turns/the same turn worth more fightiness would really ask the player to genuinely go off like an orc to get the most out of the mechanic.
Finally - use these new mechanics as a foundation setting for the research tree. Research unlocking what units can be recruited for fightiness, reducing how much fightiness is needed to recruit, giving bonuses to fightiness gains from sacking/raiding, giving bonuses to sacking/raiding relative to how much fightiness you have, boosting the stats gifted by fightiness. The sky is the limit really.