What could have kept the game from being legacy and be popular?
Not being a dummy map painter intended for office multiplayer sessions, a shell shipped with zero depth or flavour, would've helped.
The biggest problem with Imperator at release was that it contained a floor base for everything a potentially greatest strategy game in the world could ever need...and it was bad at every part of it.
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It could've had a detailed political system (more detailed than any other PDS game, and I'm not even talking about 2 consuls thing). Instead there is a wonky, sad mess of a system where you manually appoint doctors and scientists in republics (or else their families get ANGRY!!11!!1!!), republics feel and work exactly like autocratic monarchies or barbarian tribes, just with unnecessary extra steps. The few events that drive politics aren't enough and sometimes don't connect well.
Its not like politics (which were a big part of the era) matter much in the game. Civil wars are hardcoded to end your game for no apparent reason. You cannot play Julius Caesar, because remember guys, Rome vanished into thin air when he rebelled and won. Somehow, the boring and broken EU4 civil wars/revolutions are more enjoyable than what we had here.
There could've been a fine diplomatic system. Things were looking up, due to how good the diplomacy and subject systems in EU4 are. Then the game released and we got a "diplomacy system" that was essentially dead on arrival. It was never improved because the focus shifted on giving Greeks all the mission trees lol.
There could've been a more detailed religious system. Instead on release we got "click button to gain modifier every 5 years". And it couldn't even fail and set you back like it could in EU-Rome. Very thrilling and feature isn't it. And the rework made it better but only slightly (how exciting it is to see Greeks burn down Delphi, right).
It could've had an economic system. Instead, there is a jarring clickfest trade system whose entire purpose is to stack modifiers. They did add auto trade in 2.0 thankfully, but I personally haven't seen it work so far (will have to look up more about it I guess). And there is no Silk Road, Indian imperial roads, Northern routes or great Mediterranean trade network that states of this era relied on.
The game at release was a manafest, which is hilarious because the way it worked was bad even by board game standards. There were 4 (four!) buildings that you could build at most. The pop system eventually got worked into a much better thing, but at release it was just unplayable nonsense.
There was no flavour. Playing a Celtic tribe in the far corners of Britannia is same as playing a Scythian nomadic warlord in central Asia. Playing in India is same as playing Rome. Out of 4 DLCs that came, 3 focused solely on the Greeks and giving them mission trees (a flawed feature in comparison to, say, the idea behind Victoria 3's journal system). The rest of the world never got the spice it needed. No wonder people began abandoning the game rapidly.
And unlike games set in later eras like CK, EU, Victoria and HoI, this era doesn't have large scale "familiarity" factor to save it from the lack of features. Even with EU4's kinda ghastly and boring mechanics, people know nations, cultures, city states and the fun alt-history gameplay of taking a minor state and turning it into an empire. EU4 can survive on that alone, despite having depth of a puddle. Imperator couldn't, because the far more detailed map of tribes, kingdoms, empires and cultures couldn't make up for complete lack of any interesting mecahnics and flavour.
Then there is the community legacy. CK2 is still going strong after a decade because its depth and great feature-set created a wonderful modding community that enjoyed working with the huge number of features the game handed them. The mods keep the player base hooked and people keep coming back to play and try more. Similar thing can be said for EU4 (which I keep around only for mods like Anbennar/Imperium Universalis/Voltaire's Nightmare now). Imperator didn't have enough features to make even mods interesting beyond a certain point...and there were instances like the guy who made the wonderful Bronze Age mod abandoning it rather too quickly.
For example - a late antiquity "Fall of Rome" mod would've been a wonderful setting if nomads, barbarians, famine and diseases had any halfway decent features in the game. But Imperator lacks those, so those mods and its playerbase went to CK3 and are thriving there instead.
Most importantly, Imperator could've had a potentially great character system with huge ideas for roleplaying, intrigue and such...that came out completely dead, and got borked even further with patches when the major/minor family system was introduced. There are just too many things wrong with it to list here, visually and mechanically. It didn't need to turn into CK, but it could've easily gotten close with already existing mechanics and event chains, and it never did.
And personally, the dead character system was the main reason I mostly stopped playing Imperator.
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Imperator was set up to be inferior at almost every element, so why would the game survive indeed?
The end result? People who enjoy map painting tried it out for a bit and went back to EU4 and HoI4. People who enjoy deeper, more meaningful gameplay and/or roleplaying, went back to CK2 and Vicky2. Player numbers hit the floor within a few months back in 2019.
By the time it began picking up steam, it was too late and the game was abandoned.