TWWH introduced the inflation mechanic for army upkeep, the more armies you have the costlier they are. I find this is a better limitation than hard limits in R2/TWA since it feels more diegetic rather than gamey. There's however still a threshold in the midgame where, once you've broken past it, you still end up swimming in money and owning more settlements is almost always a net boon to the player, so a large, well-developed Empire is practically unassailable.
So I thought about it, what if the economy of the player as a whole was subject to inflation like in RL? The more money you have circulating in your empire, the less it is worth. So my suggestion is that not only does army upkeep go up with every army, but let there also be a slight increase to recruitment and building cost and a reduction in effectiveness for money generating buildings for every X settlements the player takes over. This way the player would still have to be thoughtful about where to spend his money in the lategame and a large player empire would come with its own set of challenges instead of becoming an unstoppable juggernaut that's boring to play as. You could even add research and building effects that affect the inflation rate. I would even exempt trade income from this to make it more useful.
This should however be a player-exclusive mechanic. The AI already has a hard time keeping its large Empires together should it manage to create one.