in my opinion, the current pledging system feels kinda limiting as far as practical ways to hire units as the only really cost-effective is to hire pretty much exclusively form the faction you pledged to.
1. I purpose to introduce a slightly altered pledge discount system it would work as follows if you are pledged to a particular faction you would get the pledge discount for all of that factions allies in addition to just the faction that you pledged to. for example, if you pledged to Durin's Folk you would be able to back up your army with some Dalish Archers without the pricing being the roof ( talking about larger quantities of units )
2. now I also think that a player that is pledged to an allied of a faction should not be able to hire the units that require a pledge to said faction examples: Elf warriors, Dwarf warriors, Gondor towergards, Uruk berzerkers, Mordor's Olg-hai ect. these units should still only be available to players who pledged to that specific faction ie. you would not be able to hire high elven warriors if you pledged to the Dúnedain of the North (Rangers) even though they allies. in short, units that require pledging to a specific faction will stay the same.
I feel like this would fit into the game fairly well as you can already do conquest for allied factions and it would make sense that allied factions would team up to help each other in different battles it also seems like it wouldn't be too hard to implement on the coding front (not that I know anything about coding though).