Profession: Magical Researcher.

The Magical Researcher, if inside a city, will increase the essence output of Magical Academies by 50%, and increase Spell Point acquisition by 50%.

This makes him the ideal Garrisoned Governor of a City with many Magical Towers and Arcane academies.

Being a Magical Researcher also makes the Magical branch of the tech tree cost 4/5 the Knowledge points. This means that your magical research will be 20% faster than other Sovereigns.

While never as Intelligent or Wise as the Field Command Sorcerer, the Magical Researcher has more access to Essence and Spell points, ultimately meaning that they can cast more often from a wider variety of spells.

Meanwhile, the Adventuring Wizard will have greater Wisdom and Intelligence (usually) which means his spells will be more powerful, and his mana will recharge more quickly.

Another potential use for the extra essence provided to a Magical Researcher (if the garrison of a truly potent magical city), would be a greater flexibility for the creation of more magical items and a flexibility to imbue more champions with essence. Therefore, if you choose this path with your extra essence, you will have many more magicians that merely yourself ... and you can send these mooks to the far corners of the Continent to do your Exploring for you.

Ultimately, Magical Researcher is the profession for a Sovereign that never wants to leave his home. A rare choice, for sure, but not inherently a weaker one.

One thing I would put out there is that since essence acquisition is so central to the sovereign, essence should only be gained from his physical location (ie a direct transfer).  So while those buildings and artifacts produce essence, they would only produce essence when a channeler was physically present to absorb it (while they would be producing essence the rest of the time, it would be going to waste as no-one who could channel it would be present).  This would both reward any channeller (not just a magical researcher) for being in a city (instead of only being there to heal), and keep things a little more balanced.  Why should a big empire gain more essence and more magical power than a small empire?  After all, that big empire already has an edge when it comes to the military (sheer numbers) and the economy.  Why make it more powerful magically as well?  If one of the goals is to limit city spam, then I should be able to hole up in my one city with a couple of attached outposts and focus on magic, or adventuring (sending strong hero's out without quick expansion), and be more powerful in the area I choose to focus on (magic/adventuring) then a nation that is focusing on aggressively expanding their empire through city spam or conquest.  If by aggressively expanding your empire you automatically become superior in essence and magic, and oh incidently your hero's become as powerful as a nation focusing on adventuring because it is easier for you to adventure with broader borders, then suddenly this becomes a city-spam, whomever claims the most territory first wins game.

I thought that is what we were trying to avoid.

The problem that I see is that when you come up with a solution for balancing a stay at home channeler to a channeller who is out fighting you then eliminate the balance by enabling the sovereign out fighting to get the same benefits.  There needs to be a more academic, less combat oriented way to improve your sovereign.  Otherwise every player will be forced to play their sovereign the same, and where is the fun in that, from both a roleplaying and a strategy standpoint?  Now weighting a sovereign staying in a city built for magic seems like a decent balance to a sovereign who is fighting and exploring, but then giving the same (or almost the same) capabilities to a combat channeler defeats the purpose.

Forcing all essence generation into one city mitigates the whole "larger empires become stronger at everything" problem, but it doesn't help the whole "sovereign adventuring and fighting becomes stronger at everything, even things not particularly combat related" problem.  Because a channeler can have a city generating essence, and gain essence leveling up, while another one just has the city, is just as imbalanced.  Now, having a "Magical Researcher" kind of a step towards balance.  The problem is that now you are locked in.  As an opposing player I know you are going to stay in your cities, and you as the player are going to be forced to stay in your cities, because that trait is powerful enough that if you choose it you have to play a certain way.  Once again, we get the problem that people are locked into strategies.  Now if I'm a channeler that specializes in assassination... I know where to find you.

A game I felt modelled this perfectly in style, if not in game mechanic, was Dominions 3.  You chose whether or not to have a combat pretender, enhance your positive dominion attributes, or have a wide selection of magic in addition to a number of other less well used possibilities.  You could use your pretender for a number of different things, and how well it complemented your chosen faction determined how well you did.  This meant that you did not have to employ your pretender in any specific way, and while a combat pretender was good at combat, he/she/it wasn't necessarily good at anything else.

Essence is so powerful, or has the potential to be so powerful, that monopolizing it to any one playing style just seems unbalancing.

I am not monopolizing essence. Adventurers can gain essence by sacrificing the rare artifacts that they could use in other ways. THEY COULD USE THOSE ARTIFACTS IN OTHER WAYS. Warlords gain essence by leveling up (THEY COULD LEVEL UP IN OTHER WAYS) ... meanwhile, anything given to a Sovereign SIMPLY because he sits at home is a non-choice and thus some-how broken.

Is this for a mod you're making?

Its not "planned for a mod" but may become part of one. I like your changes to "one time" gains to essence, only the rarest of the super rare... however I like all of my Long-term Essence gains Ideas.