I came with an idea. Instead of creating Legendary Heroes make Legendary followers that would not require a model and voice actor but would add an interesting asset to the game.
So Warhammer got a load of interesting characters that a lot of people would like to see in the game. With this simple idea, you could add them all with less cost, time and effort.
So how does my idea work?
First, you need to complete a quest in order to get 1 LF ( Legendary follower ).
Let's go with Kurt Helberg.
If you got him as a follower you get an effect - whenever you are losing you will get a reinforcement of Reiksguard Knight ( Maxed ). Additionally, Reiksguard gets some kind of small bonus.
So a follower for reinforcement that you can add to every single faction to the game ( Korhil and White Lions, Drycha, Komrad )
This is just the basic LF that every faction could get for securing there own land ( or for destroying completely 1 faction )
So we got the first type of the LF that is the reinforcer.
Shadowblade - complete X assassination in the game and shadowblade will join you as a follower. With a simple effect. Assassins in your army get + X% to assassination chance or some other bonus. If you want to be more hardcore with the assassination whenever some fighting conditions are made you can summon Shadowblade who can Pew Pew and enemy General or Hero.
This way you can add Mathias Thulmann or Bruner etc.
The next idea is creating an LF for multiple factions like Gotrek & Felix or Orfeo.
They can join you for X turns and then they leave.
With the LF you could even do Nagash. As Arkham when you recover black pyramid you can get as a follower spirit of nagash. Then the more books, you get the stronger his ability is ( For example revive a unit or summon a unit, etc ).
Get 9 books and a battle and resurrect Nagash. Who would be just a lich. With more quests of regaining his artifact and hay lich, nagash is evolutiong to the Nagash from end times with some unique effect on the battlefield but still just as a follower.
Technically you can add every single missing character in the game and just in the future remove him and add him as a proper LL.
One last thing with this idea. Faction leaders can have maximum 2 of LF and other LL just 1.