Good day! This suggestion is one that has been weighing on me for some time, and I believe it could cause controversy. But let me begin!
In the books, Tolkien doesn't once mention plate armor. Never. This is undebateable. Every armor he mentions is leather, chainmail, scale mail, lamellar, and possibly a Lorica Segmentica (which is still not plate). For more proof, please read these articles by renowned, even-more-nerdy-than-me (I didn't think it was possible either) Tolkien scholar Michael Martinez.
(Note that Martinez makes several mistakes, there are more types of armor mentioned, and Imrahil's bracer is definitely metal)
Now, to list armors in the mod that "breach" the No-Plate rule. I will also list ideas and post artwork for canon replacements.
1. Gondor: The Gondorian infantry wears bulky plate right now. Of course, this is not what they wore in the book. Here is a quote. "A cloak of dark green over a cloak of fine mail". Also, in the book, it is said that the "Black and Silver" with the White Tree and Crown is unique to them. Therefore, here is my proposal.
A grey, black, or blue tunic with a silver coat-or-arms shaped shield. This would be blank for NPCs and maybe have customizable colors for players. The helmet is shaped like a movie helmet with a silver star.
2. Dol Amroth: No, I'm not going to make you remove the Hussar wings. But I do think that they should wear different armor. A blue and silver tunic with a swan emblazoned on it over grey mail would work.
3. Thought this doesn't strictly break the "No Plate" rule, I wanted to bring this to your attention for the Rangers of the North. A little apart the Rangers sat, silent, in an ordered company, armed with spear and bow and sword. They were clad in cloaks of dark grey, and their hoods were cast now over helm and head. Their horses were strong and of proud bearing, but rough-haired.... There was no gleam of stone or gold, nor any fair thing in all their gear and harness: nor did their riders bear any badge or token, save only that each cloak was pinned upon the left shoulder by a brooch of silver shaped like a rayed star.
4. The High Elves: The armor of the High Elves has some parts that need converting to chainmail. I suggest something like this , but keeping the Last Alliance helmet, because that helmet is iconic and awesome.
5. On to the Galadhrim. I like their current armor, but I think it would look better with a grey and gold surcoat over grey mail. The helmet stays the same. Also, there is a book quote for a possible new unit, the Guards of Celeborn. "they were tall and clad in grey mail, and from their shoulders hung long white cloaks."
6. The Uruk-Hai of Isengard: These guys I believe would look better wearing leather tunics over crude mail. Pretty simple. And also, they should have a white, angular C-shape (the S-Cirith) on their helmets, and use bows and stabbing swords. They can keep their iconic helmets.
7. The Uruks of Mordor: These should look similar to the Uruk-Hai, but with the current hemet and less leather. Gundabad Uruks would be similar. I also believe both Uruks should be debuffed to the strength of a Gondorian, which is more canon, but they should also be cheaper. The Black Guard of Barad-Dur if added, could be current Black Uruk strength.
8. Orcs in general should just wear less uniform, mildly strong looking armor. A few scraps of leather or scale, with a chain shirt, would probably be the most a normal orc could afford.
9. The Dwarves of Durin's Folk: These had unique weapons and armor. "In battle they wielded heavy two-handed mattocks; but each of them had also a short broad sword at his side and a round shield slung at his back. Their beards were forked and plaited and thrust into their belts. Their caps were of iron and they were shod with iron, and their faces were grim." Obviously, the weaopns and armor of the Iron Hills dwarves should be changed to match this. The Trimmed armor should instead we changed to Gilded armor-chainmail gilded in the repective metal.
10. Ered Luin: The Dwarves of Ered Lun should wear similar (but blue) armor, but also terrifying, gilded facemasks. All Dwarves should have moderate fire resistance, and maybe wearing a full set of Belegost Armor (gilded Blue Mountains gear with mask) could give moderate resistance.
This Artist has done many excellent and canon drawings for the M&B mod The Last Days of the Third Age. I reccommend you check it out.
Thank you for reading my suggestion! Let me know if I missed any factions. As always, comments, Kudos, and constructive critiscism is welcome.