That’s a pretty good question, and I don’t remember it’s been discussed so frontally before your post. But, funnily, I think this meddles lot of things already discussed since months and years 
For instance, yesterday evening I’ve read the very interesting answers by backers to this tweet by the devs yesterday evening. I think this is closely related to your very question, as this is basically a list of what people hope to see in A.C. It could be partly summarized this way: players want to see their settlement progressively build up and expand, they want to see this evolution marked into the scenery (with paths for instance), they want the game to be challenging (although not too much), they want to build still bigger monuments, etc.
Please note everything I’ll write hereunder is a very personal opinion, that may be contradicted by the devs themselves because of technical limitations, or if they don’t feel up with implementing such features, or that could come later with upcoming DLCs, etc.
But, honestly, I don’t think the game will get boring for a number of reasons:
1) Build a society
In the official announcement has been announced traditions will appear as soon as the beta – I don’t expect them to be fully fledged for the beta, nor for the final release, but at least that’s the proof the devs will give importance to that aspect of the game.
In my view, that’s the single better point, as this means you’ll be able to forge totally a society, from scratch. But with passing time you’ll be confronted to other people, with different traditions. My hope is that if you choose any radical path, this may hinder your diplomatic relations–and as a consequence your trade relations, necessary to get resources you don’t produce. This would make the game alive, as it would oblige you to take decisions, without knowing what the future will be.
For instance, if you see one tribe gaining control over your whole area, will you try to resist them, or embrace their way of life, their new gods/spirituality and customs, just to be able to keep good relations with them? If you’re pretty adamant that it’s totally your advantage to have women authorized as hunters and chiefs (as this means you’ll have twice more good interesting candidates for such functions, making them far more efficient), will you accept to refuse women in such positions, just for the sake of getting flint through trade with those powerful neighbors? Such a single feature needs pretty drastic reactions and decisions, and avoid any boredom if your 50-hours settlement is not to be fatally hindered by the in-game situation.
2) Don’t build anything you want as you want
I think also you should not be able to build anything anytime. Let’s say A.C. is crappy game: then, you should be able to build Stonehenge as soon as you get the “knowledge level” for. Just, this will need 50 generations of work instead of 10 if you dont have enough resources and people. I mean, why not? But this would not be real fun.
My opinion is I think there should be a jauge on what you’re able to do, the lone jauge that allows you to build bigger and loftier buildings.
Let’s call that jauge “prestige”. It doesn’t need to be the hassle “with 10K prestige level you’re able to build that”, if that means just piling up prestige. That’s really not natural, that’s been seen far too often in video games.
Instead, it could be done very progressively, from tiny objects like necklaces made with boar teeth, to magnificent jadeite axes. If for instance you evaluate the prestige given by such tiny items, a cairn, one sacred source, tree or rock and 25 people in your settlement, you should be able to build an enclosure measuring for instance 30 meters. That’s not enough, you’d want a 50 meters enclosure with a real front gate, to get more prestige and build a real covered alley to bury your current tribe chief? Ok, let’s go to work. Make more cheese, trade it for shiny jadeite axes, build more menhirs, then you’ll have your enclosure and front gate. Attract people in your sanctuaries for sacrifices, invite neighbor chiefs (in the condition you’ve got enough food for all of them), progressively pile up more prestige over time, protect those prestige items from raiding neighbors, until the moment you may build the inner circle of Stonehenge, hoping no neighbor tribe will beat you on the prestige race and build a whole Carnac menhir field while you’re still building your first one–or you’ll be condemn to just rest in a backward settlement, which would be an unofficial game over.
For later times, let’s say the game reaches a development phase like Near East Bronze Age, or Greek/Roman eras: you could build bigger palaces, adding room after room, so that you slowly become the regional capital. Achievement and breaking lines is fun. And failure may be fun also, if you think you may do better the next time.
3) Adversity and challenges
Lot of people in the twitter answers told they want a challenging game – although that’s quite a thorny point when you think about it. Does this mean they want a tornado or a flood every 5 or 10 years, destroying half their settlement? Neighbors raiding you every 2 years, that raid half your people every two years? Epidemics?
All of that may sound a bit boring if coming too regularly. But that a fact that if nothing happens in game it could become really boring; and it could be boring if such adversity comes too regularly, and/or with too drastic effects, and/or never come.
But lot of things may happen that oblige you to redefine your plans in-game. For instance, if you imagine raids not be Neolithic World War 0, but something a bet? I mean, when looking at ancient warfare, it’s pretty assured by now most wars were a show of force, while diehard battles seemed to be quite rare.
So, what if you define the amount of loss and gains if you’re the winner or looser of the day, just by making a show of your force to your opponents, with magical gestures, savage yellings and impressive warrior dances, evaluating if your enemy is that much ready to fight you, hesitating yourself if you should still escalate the situation?
If they’re twice more people than you, maybe you’d better give up and give them the 3 wives, 5 cows and a number of axes as the tribute they ask, and go on with your hurt pride.
If you feel you may get the best, it could be a good thing tempting everything on a bet, make a real show of force, discuss with them as real men on the battlefield and try to keep everything, so that you enter a shiny new era with all those resources you gained with your prowess.
Aside of such obvious topics as warfare or calamities, we already now (from the official announcement and other topics) that there will be also a number of adversities in your games. The bigger your settlement, the most efficient it will have to be regarding resources collect, tool production, etc.
Hopefully, if you’re along the sea or along a river, on a harsh mountain full of precious jadeite stone that you may trade for food, the challenges will be very different.
Aside of that, I think a good deal of fun could come if for instance your river doesn’t produce as much fish every year, or if your flint mine may get exhausted, or if some neighbor tribe flood the market with the rarest stone axes ever made, that grant more prestige to their owners.
Then, you’d be obliged to redefine your plans if you want to avoid stagnation–and redefining plans to adapt to any new situation is definitively not boring.
4) End-game crisis
I think that’s the last point, but there is also another aspect that probably will be added to the game one moment or another: Indo-European migrations (a definitively brilliant proposed by @Tim), at the end of the game.
We can’t say for now if that’ll be implemented, but for sure the devs seemed to be interested. This means you’d have, at the end of the released game, something like an “invasion”. It could be a pacific invasion, with just a few more neighbors in the area, maybe some of them coming in your tribe with new customs, or maybe wild savages only interested in looting.
Whatever the way you see it, this would mean at least the final release of the game would have a major achievement: face those new people coming en masse around you, and maybe among you own tribesmen. Depending how you managed your game before, you’ll have to survive–be it culturally, militarily, diplomatically. Will you accept them and trade? Will you resist and fight? This may depend how to dealt with the game before, what was your past story, how you chose to face adversity and challenges before.
And, thinking about it, is it that much different to the Celt situation facing Romans, be it in Italy, Iberian Peninsula, Gaul, Germania or British Isles? Such an invasion/migration for sure would be a challenge, a real goal at the very end of the game, that could drive you to try multiple strategies.
5) Mods
Mods, also. They could change you whole vision of the game, in case you’re bored with time.
Just imagine anyone creates a mod where you’re a Bedouin tribe in the middle of a desert. Lot of trade and raids, no real resources save from a tiny oasis. That could be quite a challenge.
And what if you’re a Siberian tribe, facing the harsh conditions of the arctic landscape?
Or what about a Native American mod? A migrating Great Plains tribe, with the possibility to recreate the Mound Builder/Mississippian civilisation? Until the moment you see goddamn palefaces showing up and tell you they came in specially for you, as a special privilege, to get the chance to embrace a true civilization…
(hint: I’m not a modder myself–just to be clear)