I don't like the "decadent empires" meme, since it really sounds like someone taking The History of the Decline and Fall of the Roman Empire as gospel truth when we have a much better understanding of Rome than Gibbon did back in his day. I'm more a fan of using the decadence concept to explain the cycle of nomads conquering a people they perceive as weak, coming to rule it in exactly the same manner as the conquered did, then being overthrown by a nomadic group themselves; you can't both have nice things and a warrior society—guns must come at the expense of butter.
To make empires naturally decline you need a steady internal pressure, and the diplomatic game has to be fun. Let's say I want to conquer some of my neighbor's land. I'm large and powerful, so I can surely win the war. Conquest takes soldiers though, and soldiers take money. To pay my soldiers I've necessarily been taking a fair amount of taxes from my empire. My home culture doesn't mind so much, but the people I've already conquered? They'd love to see the bulk of my forces off somewhere else. If they storm the citadels in their cities and lay siege to the forts I've built in the area, restoring order will be quite expensive for literally no gain—I'd simply be keeping hold of what I already had. If an enemy in good health sees that my forces are off somewhere else? Even an empire can lose territory, and they'd have a much easier time besieging my lightly garrisoned forts and cities with no army opposing them than I'd have trying to retake them, fully garrisoned, with an enemy army in the field.
So I have a few options. I want weak enemies, it's true, but ideally my neighbors wouldn't hate me in the first place (though as a conqueror I can only do so much for that). Negotiating truces (like how you can exchange hostages in Sengoku) and supporting claimants should probably be in the game. Depending on their government type and your strength, you might be able to lean on the right people and get a satisfactory claimant in without a fight. Of course if it comes down to a fight, succession wars can be some of the longest and least profitable. If I'm a real jerk, I might try to get someone to attack the person I'm really worried about, or at the very least intrigue to keep them too worried with domestic matters to take advantage of my distraction.
Internally, I've already mentioned that I have to worry about rebels. I really like Victoria 2's POP model, and any game that wants to realistically portray empires needs to have something very similar. Managing my empire should get more difficult as it gets larger, as conquered people are less happy about paying taxes but require troops to keep down hopes of rebellion all the same. I like what EU4 tried with the ability to temporarily increase local autonomy to keep unrest down, but it decreases automatically as does the reason you probably increased autonomy in the first place (nationalism). Clawing back autonomy you grant to the provinces should really upset them, make future autonomy grants to them less effective, and generally tie up some kind of "administrative bandwidth", which would also be an important general concept: some government types just are not good for administering large empires. Bureaucracy is expensive, and if you appoint big men to govern large areas for you to allow you to govern more territory then you shouldn't be surprised to see governors declaring their independence every time you have a succession, which means more money spent holding on to what you already have.
The ossification that tends to happen in large empires would be really hard to implement. Pressures need to make someone who's losing have an easier time adapting and improving than someone who is "winning". The best way I can think of would be to increase social unrest in POPs, and take more administrative bandwidth, to push through reforms when you're on top. A player will know that they need to do this, so it needs to be finely balanced so that they always feel like they're advancing too slowly. This way they're in danger of pushing it too quickly and having major revolts on hand which their elite military units (usually near the top of a given social order) join, but at the very least they'll start getting handed some very real external defeats (losses to internal rebels would have no effect on reform approval, but maybe loss of conquered territory to native rebels would have some effect) to speed things along. Of course once you've started losing territory, administrative bandwidth you're spending on reforms is administrative bandwidth you're not spending raising armies to counter-attack.
Finally, I think a game needs to have a way to represent the fact that there is constant low-level fighting in an area. There are no armies rising up for you to smash, just groups of people hitting you wherever you're weak and then fading back into the wilderness or the populace. Remember, soldiers are expensive whether you're fighting an enemy army or a bunch of insurgents, but if you let the insurgents run wild then you don't exactly control, or benefit from, the territory.
So basically you need POPs from Victoria 2 to represent the populace, simplified characters from Crusader Kings 2 to represent important people, and the will to tell the player that "no, the game mechanics don't make it feasible for you to conquer the world."