As a player who needs some kind of ingame goal to keep me playing after all previous goals or new things have been met or built, I feel a lack of motivation to keep playing a map after some point in Tier 4. What I’m proposing is a multi-fold Victory system, where you can achieve Stars based on multiple elements in the game.
These Stars can include things like Military, Resource, Food, and Riches. The Stars consist of missions that can be preselected when choosing the parameters of a new game, or randomly assigned. There are also opportunities to have things that give multiple stars at the same time (an option could exist to allow or disallow this on Random Stars).
Ideas for Stars could include:
Military: Survive the Horde (a massive Raider attack that you provoke), Enemy Fortress (big Raider encampment that will require a massive Military to attack), Ten Years of Hell (powerful raider attacks every year for 10 years)
Resource: Build a Wonder (large building with a very large resource cost, also seen in Age of Empires), Supply An Expedition (Initial resource cost with large ongoing costs for X years, perhaps with some kind of building to accept the resources)
Food: Sacrifice of Sacrifices (Requirement of each livestock animal each year for X years, thus draining you of butchered meat), Fine Feast (put together a number of fine foods like Cheeses, Preserves, etc),
Riches: Fill a hoard of gold within X years, buy X amount of Jewel item from odd traders
Combinations could include:
Defend the Wonder (Military + Resources) - Mixes it up so you have to build a defensive perimeter around the Wonder before it’s built, and either a Horde or 10 years of powerful attacks will attempt to destroy the Wonder (instead of your Town Centre)
Build the Jewelled Wonder (Resources + Riches) - A Wonder to be built which has a lot of components only held by traders, and at high prices
When achievements are introduced, there can be achievements for each mission, and also for getting stars, based on difficulty level (with an additional achievement for number of Stars in a game if it was done on unmodified settings). These could also be bragging rights and ways to show how well you have played the game.
Of course, nothing is stopping you from continuing after continuing to play after or without getting Stars as normal.