Got an idea (eras).

The default setting is in favour of the player (relatively easy conquest with some protections like "if a race falls below 10 planets they become super desperate and aggressive" and "if the player controls more than 50% of the galaxy everyone is angry" but typically just passive & lazy aliens).

Then there are 5 eras (A,B,C,D,E) each era lasts around 50 turns. The player is told stright up a new era has started (also can see the "time till next era"). Each era presents a unique challenge/danger the player has to face and overcome (rebellion, a new alien race spawns and attacks the player, major races form a coalition to fight the player, etc), the last era always have "invasion of aliens from another dimension" and once they are dealt with the game ends and your success is measured in the prestige the Empire accumulated during the whole game. Also slight different rules and options and opportunities might be presented in each era. If the player deals with the era problem before the era ends (like crushed the rebellion faster) that's fine, the rest of the era time goes on the default (favourable to the player).

In addition during each era one major AI race is selected as "Glory of RACENAME" and they get +100% to military and become more ambitious (so the major powers can change over time) and one as "RACENAME became decadent" and they get a penalty to military since they use up most funds on booze and chicks :)

To make it more interactive during each era the player is allowed to pick one ambition (optional), it allows to trigger some event. Like if the player wishes to "crush pirates" a big force of pirates will be spawned and the player will fight them (in addition to the main danger/challenge of the era) but once they are crushed the pirates will target the player(Empire) significantly less frequent till the end of the whole game. Or the player might want to make a non popular reform and will need to face a minor rebellion but then get the benefits.