My suggestions for how the companion system can be added to/reworked/modded:
1: Add loyalty system to companions
It would be very cool to be able to raise and lower your relation with companions in the same way that you could in warband. You would raise/lower your relation with them by doing things they agree/disagree with: too low of a relation would result in them leaving your party while raising relations would open up new functionality. I envision it is that companions start out as being nothing more than extra soldiers that you can outfit. We can call this loyalty level 1. Then by raising relations with them, you can raise their loyalty level with you and unlock more abilities. Here's a rough draft of what the bonuses could look like:
Loyalty Level 1: Companion join party
Loyalty Level 2: Companion unlocks ability to lead caravans, execute quests, and take on party roles
Loyalty Level 3: Companions can become vassals and governors and lead their own parties
Loyalty Level 4: Player can switch between this companion and himself during battles. This would allow the player to take over as this companion in the event that they are downed on the battlefield.
Loyalty Level 5: Companion's Party Leader perks are transferred to the player while they are in the party. Companion can succeed the player's clan role in the event of their death.
2: Add unique characters with unique backstories, skills, and quest-lines.
I understand that unique characters wouldn't work too well given the aging mechanic and perma-death but perhaps this could be used in a mod. I would love to have all of the companions have a rich and unique background which the player can explore over time. If paired with the loyalty system outlined above, raising a companion to the next loyalty level would unlock a new quest associated with that character's backstory and in turn help the player gain relation with the companion even further. This could definitely help remove some of the grind aspect in the early game as you would complete meaningful unique quests which benefit you rather than fight bandits non-stop and complete cookie-cutter quests.
3: Add different ways to find companions other than taverns.
It feels disingenuous to hire all of your companions as mercenaries in taverns. It would be great to spice it up a little. For instance, you could randomly find a companion as a prisoners in a bandit party. If you defeat the bandits, the companion can join you. Similarly, maybe a companion starts off as an outlaw in a group of bandits. The player could then defeat the group, capture the companion, and then convince them to join the player's party. Another idea is to have some companions join you when you remove a gang from a town or take on a certain type of quests. This would add a little spontaneity to the way that companions are found. The way that you find them can also be tied with their backstory/skills.