Ideas on Cohorts.
I have been thinking on cohorts a lot recently. They are are an incredible way to show a Character becoming important over time. Just as important as any class feature or magic item. They are who is important to a character as much as the other party members (and often more so).
If you want to have them feel important I recommend a few things.
Don't Let them Be Faceless.
The 2e AD&D DMG has this to say
When the player is as concerned about the welfare of the NPC as he would be for a normal player character, that NPC can be treated as a henchman.
This seems superficial at first but having used a half dozen different methods of acquiring henchmen/cohorts/thegns in games this one is the only one that has stuck out to me.
Players should care about their henchmen. They should also have a reason the henchmen would follow them besides something like "I am rich and legendary" those make followers but not loyal ones.
A player should be willing to risk their character for their henchmen. If they always push the henchmen in front to take all the risk and try to cheat their henchmen out of their shares then the henchmen would be unwilling to take those risks and would look to cheat the players character too.
The Henchmen have expectations.
Henchmen are loyal followers to the character not the party and not necessarily each other. If that character dies the player may pick one of them to be his new character. It provides an excellent reason for the new PC to be there however if you have 2+ then any treasure that was left behind by the PC including magic items should be split up amongst those henchmen and some may leave the adventuring party permanently. The magic items don't just go to the party members. The henchmen would likely demand them and if a party member tries to keep them maybe they suddenly have made a bitter enemy in their wake.
This should not be their end. You spent a long time building these characters and they are noteworthy to the party if done right. They may appear as new rivaling adventuring parties seeking to get that treasure the party knew about but did not adventure for promptly. They may retire to be NPCs in the local town able to provide services or contacts. What happens when a player runs afoul of the local lord and when he sends guards to arrest them one of those guards is the 3rd level former cohort of a dead ally. This immediately provides a ton of roleplaying opportunities for the group.
Give them a voice
A common criticism of OSR games is there often is little roleplaying opportunities. This is especially true of open table games like the ones I run. Players promptly leave town and things like interacting with shopkeepers and local quest givers is something that gets brushed aside in favor of dungeon delving and hex exploring.
Find reasons to have the cohorts to speak. If you are a fan of things like overloaded dungeon dice and random encounter dice you can assign 1 number on the dice as "cohort does a thing" maybe that thief follower starts marking a way out with a bit of charcoal even though the party didn't say they would, maybe one will discover a secret door the others didn't see (or waste time swearing one is there), maybe one will stop at a small shrine in the dungeon and pray at it. Find reasons to make them exist more then faceless things that only appear in marching orders and initiative queues.
There are many little things you can do to help flesh out followers and cohorts to be more then just faceless masses and doing so helps the players often get more involved in the setting. I recommend doing things with them.