Speaking in game terms, the key points to me of a crisis are that they have to matter for you, but you have to have a way to "defeat" them. Like I don't want a plague that just wipes out 1/3 of my empire and there's nothing I can do about it. But they should be punishing enough if you don't fight it. I think for balance, they should also hit the leaders more than those trailing. It's hard to do, because usually anything you have that counters an action, someone in better position is also better equipped to fight it off. Like if I'm running away in gold, buying plague doctors is really easy. Or one game where I was in great spot in the crisis, I had the barbarian invasion and I was just overflowing in troops that I just used the new barbarian camps to farm XP and it cost me nothing.
I do think that they eased off some of the crisis damages, probably the same reason why they eased off the dark ages in civ 6. Even if you leave the current mode as the default, having a "Dramatic Crisis Mode" where you can really get hit hard might give some people more of a challenge. Or you just do extra scaling to the difficulty level, so people at deity just get hit hard. I guess if the barbarian invasion troops had like +15 combat strength against Deity players, that would at least take some concentration in fighting them off...
The problem with any 'plague' mechanic is that prior to Venice's invention of 'Quarantine' around 1600 CE and Pasteur's Germ Theory of the late 19th century, there wasn't much you could do about Endemic Disease except bury the dead quickly and pray.
That means any in-game such mechanic has to be 'nerfed' or tweaked. Getting hit by two pandemics that each remove 1/4 to 1/3 of the population within a century is, in practical terms, Game Ending - as it largely was for the western Roman Empire.
But only 'largely'. There were other systemic problems with the Roman polity already, in their tax system and individualized loyalties that made generals unreliable and kept the Imperium up for grabs every time an Emperor stumbled. Doing anything about depopulation when the government is almost constantly unstable makes for a poor set of response to the Crisis and a very Unfun game.
But, the entire Crisis period can be made gameable as well as Serious. Any Crisis is not just a single event. It is made worse or less so by the existing structure (Government, Civics) of the Civ affected, in its neighbors, and in the specific actions taken in response to it. - Which, in turn, may precipitate another Crisis just down the road that is worse or completely different.
Given that Human players will inevitably game the gameable, Crisis periods will have to be balanced both between what the AI is capable of and human players are as well as between being serious enough to Force a transition to a new Civ and being so traumatic the gamer rage-quits every time.
And that last may be most important, because it addresses what I think is the great problem with the Age transitions now: gamers want to play one Civ forever as they are used to in previous Civs - the Egotistic Fantasy that a Great Leader can take a people from 4000 BCE to 2000 CE somehow, in the face of all historical evidence to the contrary. The Crisis periods should make it abundantly clear that that concept is largely a Fantasy, but that the alternative is equally playable.
That means not only a Crisis that gradually overwhelms, but also a direct connection between what you do about the Crisis and what you wind up with: to go from playing Rome to Mongolia should require more than 3 catapults or 3 horse resources, but some major changes to Rome's Civics and Government representing a wholesale transformation of its society from an urbanized agriculture and trade-based Central State to a semi-tribal pastoral society with a much smaller concentration of population (for which, possibly Cue The Plagues?)
All of which will take a lot more work than merely 'tweaking' the existing Crisis periods.
