The following text is courtesy of Central the Eyes Watch from Many Trees, who is also the G.O.A.T!

Why do we need a Hotseat mode?
There are multiple reasons for implementing a Hotseat mode.
* opening up the multiplayer only scenario's for those who do not have a gaming group or those that have no desire to have human interaction while gaming
* creating epic scale showdowns which will actually be fought until the end
* staving off campaign fatigue

How would Battles work when multiple player controlled factions face off?
The player would be asked which side they wish to control manually. The AI will control the other factions in that battle.

Multiplayer only scenarios
Multiplayer is an interesting way of playing Warhammer III but it is not for everyone.

A multiplayer campaign can take a long time, and it is not always feasible to create a schedule that works for all participants.

Even when you manage to find a time which works for all participants, it is quite possible that work schedules change or that social engagements can disrupt your plans, causing people to cancel game night a few times until a campaign gets cancelled completely.

We should also not forget that a lot of people are required to interact with other humans on many occasions, such as work, at home with family and when going out for drinks.
For some people gaming is their escape from the real world, something to enjoy alone, it is their respite from the day to day issues that plague them, interacting with other people might interfere with that.

Last we should not ignore the the people that do not have anyone to play with, some people just don't have friends that are interested in the same games they are interested in. Hotseat mode allows them to simulate the experience of playing with friends.

Whatever the reason for not playing multiplayer is, the simple fact remains that the wonderful scenario's that were developed for multiplayer will be unavailable for those who do not venture into the multiplayer aspect of this game.
Hotseat mode would be a viable solution to all these concerns, and allow everyone to enjoy the the full breadth of Warhammer III's content.

Epic Showdowns
While a single player campaign is a lot of fun you will be rolling the dice on which way the AI preforms. You have no impact on which AI faction will be dominant.
If you are desperate to create or recreate some epic showdowns between two, or more, specific factions than you are forced to turn to multiplayer. (For example the War of Vengeance / War of the Beards)

Even if players find a gaming group that manages to stick to a regular schedule than there is still a big chance that the game will end once a player starts losing.

It is a very common practice for the losing player in any multiplayer game to surrender before they are completely destroyed in order to maximize the amount of fun they can have with the limited game time available. A Warhammer Total War campaign can take a long time to complete and not everyone wants to fight it until the end.

Hotseat mode would allow player to create these epic showdowns alone, players would be able to set up multiple factions to be dominant in their region and than create a final face off between those factions. fighting it until the bitter end.

Campaign Fatigue
The map in immortal empires is quite large, finishing it is a challenge to some people but painting the map is even more of a challenge.

One of the many challenges for painting the map is the fact that there are limited options in the late game when it comes to armies, some units will simply become obsolete. This means that you will be fighting a lot of battles using the same army compositions simply because not all units are viable in the end game.

Some factions also have few tactical options available to them, which means that you fight almost every battle with the same playbook.

These limitations of factions do make for an interesting experience, but in the long run it can become repetitive, causing players to abandon their campaigns.

While controlling allies is already a good step forward in fighting this campaign fatigue the questionable army compositions of the AI can cause issues.
Even when the AI makes good armies they are not always what you want to control.

Allied recruitment is also an interesting step forward but because of the limitations on that mechanic it will not change your tactical options too much. There are no tech or lord boosts which limit the viability of units, and with the limit of 4 allied units per army it will not alter your tactical options as much as you might want.

Having Hotseat mode would allow players to control multiple factions with very different playstyles, it would also allow players to create their own allied and enemy armies, maximizing the diversity in playstyles.
Instead of fighting multiple battles with the same armies players would be able to mix things up a bit, keeping things interesting for longer.

-Central the Eyes Watch from Many Trees

These are what I feel to be very compelling reasons for the addition of Hotseat mode.

Though there are even more besides, I think Central did an excellent job of explaining why we should all support this petition, and I hope you'll join us in making Hotseat mode a reality in Warhammer III!

Edit: Special thanks to Mr.Soul for emailing his media contacts to help spread the word.
And a special thank you to Funky Monk, for helping me make the title of this petition.