Keep in mind this suggestion is based on the current state of npcs and units in the lotr mod, as npcs are not yet added in renewed, they may work completely differently than they do right now, however here's a few suggestions on how to improve units, feel free to add your suggestions and opinions in the comments.
I'll begin with what would be in my opinion the easiest things to implement and end with probably the more tedious to make
Choosing melee unit weapon: I'd like to be able to choose with what kind of weapons my unit starts for professional soldiers (gondor soldiers for exemple, not levymen or regular orcs or dunlending)
Increased knockback and damage on mounted units : Quite self explanatory, the mounted units should be more than just faster units, pikes and spears could also deal more damage to mounts and mounted units and players if such a thing is possible to script.
Working shield and loot gathering : With the addition of shields in 1.15, I believe it is essential for units with visible shields to actually use them to protect themselves from arrows or some melee hits. Here's why, it would make a more meaningful difference between poor units (mordor orcs) and elite units (black uruks) than just different gear and HP which means nothing rn because of the new levelling system and the fact you can change your unit's gear. It would also encourage variety in army composition as right now there is no reason to just get banner bearers other than the fact they cost more. Banner bearers should be more vulnerable by the fact they can't hold a shield. We've seen it's possible for npcs to have an inventory and gather stuff with the farmhands, it would be neat for units to have a little inventory where they'd gather items on the ground if ordered to to make looting easier.
Changing Archer's weapons: Right now archers are useless against players or other units because you cannot change their gear to give them bows or crossbows with modifiers. Having the ability to change their weapon could probably change that.
AI taking weapon reach and cooldown into account : Right now, an unit with a poisonned dagger will always win a fight with a similar unit with a different weapon of the same material. Spears and pikes units will always lose for an obvious reason, they'll run straight at the ennemy even though their weapon is designed to hit from a safe distance. Since those weapons have big cooldown and less damage than warhammers, they'll lose because they don't use the long weapons only advantage. It'd be great if the AI tried to hit while staying at a safe distance when equipped with a spear or pike similar to how archers run away from you.
Improving the levelling system : No max level was a bad idea, even with exponential XP threshold, it's not hard to get npcs with 50 to 100 hp which is frankly ridiculous, it also makes it a grindfest for pvp on servers. here's how I would improve the system, keep the current XP ratio but add a max level, 20 seems reasonable to me. make the units only gain one hp per level instead of 2. Every 5 levels, the unit can gain either a little more speed, damage, resistance, sight range or 2hp.
Pledge buff : Pledging is underwhelming right now, some factions have their basic units hidden behind pledge, some pledge units are utterly useless like the angmar trolls while other like the uruk berserkers whipe the floor with everyone because of their higher HP speed and damage, every factions should have a decent elite unit and regular units for balance (right now rangers, elves and dwarves only have elite units which makes their invasions and conquest OP) new npcs or rebalancing currently existing npcs is essential so people pledge for the faction they like instead of the faction with the strongest NPC. Another mechanic I would add to pledging to make it more interesting would be pledging buffs on the player and its units unique to each faction.
For example : Ranger pledged players and units could be stealthy temporarly (right now rangers being able to hide as long as they want is very OP and stupid) Dol guldur players and units could have a 1/10 chance of dealing poison on hit (may be too op but the point is, have a buff based on the faction's theme) Wood elves players and units could draw their bows faster while the high elves would have better movements while drawing their bows or vice versa.
That's about it for now, I'm well aware most of this stuff is probably too hard to script in minecraft but who knows, after all they are remaking the mod from the ground so it's better to adress this kind of stuff before it's too late for them to improve the base frame of the mod, don't forget to add what features you'd like to see for units in renewed in the comments.