Banners are always good as long as you have more than 10 models in your army
Some scenarios they are required for VP's, so I would always have one, probably even 2 at 800 pts.
I like the idea of your theme. Too many well drilled orcs on the field these days.
Question: Do you have a mounted war priest? I think they are hard to come by and I'm wondering if its worth taking in order to tweak the list a bit.
He would be useful for fury and bladewrath but that is kind of putting your eggs in one basket. If you swapped it out for a Dragon Knight then you have 2 hard hitters.
My basic list:
Mordor Troll Chief + 6 Orcs Shield + 6 Orcs Spear
Morannon Captain + 6 Morannon Orcs Shield + 6 Morannon Orcs Spear
(If you have the models just get 12 Morannons with shield and use regular orcs with spears. If you have spare points at the end then upgrade those to morannons also.)
Amdur, Horse + 15 Warriors with Pike + Shield
Dragon Knight, horse + 5 BD Kataphracts.
798 pts.
This list has no banners so I'd probably take out a Kata and give an orc a banner instead and mix in a few two handers instead of some other troops. If you want more room, change the morannon captain to something else or even the troll.
This list has lots of big hitters, the Morannon being the weakest at S5 2A. Unfortunately you can't use the Morannon captain to heroic strike anymore and take out some might of the opponents biggest hero. If you wanted to do that you could take Grishnakh or Gorbag which hurts the theme a little but is still ok.
Otherwise you could use the Morannon Captain to team up with another hero and call heroic combats.
I would be trying to avoid the likes of Boromir, Elrond etc with Amdur until they have use up some might points. If necessary you can still shove them in the face because Amdur has Phalanx - So 3A from him + 2 pikes + banner re-rolls, 6 attacks to win the duel.
It might be worth sending the troll and some orcs against the big heroes to take out some might and if the troll dies then Amdur can step in to save the day.