I am having a bit of trouble creating the political structures of a major empire in my world.

This empire is ruled by Aesir and is called The Aesir Dominion. 

They have been ruled continuously by a single dynasty since their founding in 1291 DR (this is not canon obviously).  The general philosophy of the empire has always been expansionist and somewhat aggressive and because of this, the empire controls a significant part of the world, along with most of the major ports and trade routes.

In recent years though, the empire has been somewhat in disarray.  Popular uprisings, political strife and outright rebellion threaten to destabilise entire regions of the empire.  To combat this and to maintain control; Alfhild Godfrid - the current Empress - has ordered soldiers into the streets, with a direct order from the Empress herself to suppress any further uprisings and put an end to whispers of rebellion amongst the lower classes.

Even in the royal court, there is unease.

People suspect the Empress has started to loose her marbles and those closest to her have witnessed uncharacteristic rages, lack of thought, the inability to recall facts and a strange malevolent presence that constantly hangs over her like a dark cloud.  Any who have voiced their opinions or concerns within earshot of the Empress though, has been swiftly and ruthlessly suppressed.  Suffering punishments ranging from loss of property, land and status, to imprisonment and execution.

Where once the Empress saw friends and allies, now she sees enemies and at times seems to be almost paranoid, seeing treason around every corner.

What would you expect the political structures of such an empire to be?  Any help would be appreciated because I am struggling a bit with this one.

Warning: This is quite a lot of additional content.  I thought it was important to share this with you all, so that you could all see where I had getting too and what I was stuck on.

Initially, I had thought that the political structure would look like this

Empress or Emperor

An active ruler and not just a figure head.

Chancellor

To whom the empress or emperor usually issues orders. The chancellor is like the empires overseer and keeper of the purse.

Regional Governors

Each one is in charge of a particular region of the empire (head of the senate of that region) Each regional governor is also a general and the leader of the armed forces in that region.

Senator

There are 10 per region, each one in charge of a sector of that region. I had thought that the senate would come together formally 4 times per year, so that orders could be passed down from the regional governor. I had also thought that the regional governor would have the power to summon the senate.

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The title and position of Empress or Emperor is passed down from parent to oldest child.

I had also thought that only the Empress or Emperor could appoint the Chancellor and Regional Governors and that the positions were usually held by brothers or sisters of the Empress or Emperor. So when a new Empress or Emperor came to power, they would appoint their own siblings to those positions.

Unless the Empress or Emperor had no siblings - in which case, they would appoint other close family members. Uncles, aunts and so on (which could quite well be the existing holders of those offices) and so in these circumstances, the existing Chancellor and Regional Governors might actually get to keep their position but usually they don't.

Senators are appointed by the Regional Governor and are minor royals or nobles that hold royal favour. So they might be the beloved niece or a favourite cousin or something like that.

Senator is also the highest rank that an illegitimate child of the royal family can hold. It should also be noted here that illegitimacy is not frowned upon in this world as it is in the real world - its just that illegitimate children only have half royal blood and so they are kept on a tight leash and not allowed to hold any of the higher positions.

This is too ensure that their illegitimate half does not really have any influence over the policies and practices of the empire.

Thus, the empire has been ruled by a single dynasty (one family) since its founding.

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I also had an idea that common folk (those without royal or noble blood) could become priests or priestesses and even rise to the position of High Priest or High Priestess.

To get to such a lofty position though, you had to be vetted and approved by the senate; who amongst other things, were responsible for ensuring that the community leaders (Like the High Priest or Priestess) were right for the role.

In other words - that they would not speak out against the empire or its rulers. So in the empire, even religion is a matter of state and controlled by the ruling classes.

Illegitimate children of the royal family can also hold these positions and usually do.

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But things are beginning to fall apart

The above ideas that I had were of the way things usually are. But as I stated in my original post - things are beginning to all apart. Political structures are breaking down and there have been uprisings and rebellions.

So far the ruling classes (the royal family essentially) has managed to maintain control but what happens when the well established political structures of an ageing empire begin to fall apart - how do things change?

That is the part I am having the most trouble with. How do I show that as the existing political structure begins to fail, it is being replaced by something else and that, that change is happening organically and has been for a number of decades, even before the Empresses madness really began to show itself?

Great name for the Empire.

You could take a page out of Star Wars. Skip ahead to when Palpatine started consolidating his power. Anyone who wants that much power isn't all there, after all. You could also take a page straight out of history. Look at the way Ceaser took power from the Roman Senate.

I realize that both of these involve wars or warring. Perhaps there's a constant threat from spillage from the Abyssal plains (or an equivalent)? That way, you don't have to build a whole new kingdom as a rival. Not to put current/semi-current politics into it; too late:

You can look at the last ebbs and flows of American political media and how the war on terror is treated. Or, some of the foreign leaders taking rights from the media. When the people are in fear, they don't always make the most rational decisions. Wow. This is becoming a thing. Sorry.

In short(er): give the Empire an enemy that has been constantly at the edges. Something that is always on the verge of ruining everything without actually doing it. Sure, soldiers die in the field or out on patrol. But, that's far from epicenter of the Empire. Over time, rights have been taken away, taxes have been raised, the people are slowly becoming poorer. The Senate USED to be 10 per region, reduced to 8 or 6. The military is growing. And, it all started with a single innocent edict.

My advise would be to look to history, find similar nations or empires with similar power structures and institutions and look at how they operated/collapsed. It also sounds like your political system is highly centralised, so maybe check out some advantages and disadvantages that typically come from centralisation. 

Based on the centrality of the royal bloodline to all levels of government throughout the empire, I wonder what purpose the senates serve at all. Surely it’s not as elected representation if senators are appointed as positions of favor from the empress (maybe I read that part wrong, but I thought that’s what I saw.) In such a system, I can see locals setting up shadow governments like the US did with the continental congress. We still had crown-appointed governors when that congress formed.

Black markets would be common if taxes were high. Which would usually start getting taken over by mafias. They’d be the only ones brave enough to defy the crown, and eventually the only ones rich enough to buy the silence of the local leaders. Usually, those groups aren’t political and prefer power by wealth instead of by title.

But maybe a group or two has become more organized and powerful, appointing members to  “supervise” the empire’s official appointees, edging them out and taking over. Making new alliances with rivals of the empire (because hey, they always offer better deals to trade partners that open doors previously closed for business.) Using the military to root out rival criminal gangs (the expenses of such campaigns are charged to the empire, of course) when such things were considered poor use of military resources before, and the drain is being felt in the capital.

So anyway, there’s a few ideas from my brain of problems such politics could give rise to.

Thanks for pointing me in the direction I need to go.  Seems like I need to get some history books out of the library and read up on how the real world empires of old were structured.