The reason people complain about late game isn't because of lack of content. It's a difficulty scaling issue. Early to mid-game is very fun because it's challenging. You as the player starts out basically at zero and you have to establish yourself in order to compete with the AI computer players but the AI computer players are already established. The issue comes in when the player becomes too strong (if he develops his characters skills/attributes properly) and surpasses the AI computer players and the AI doesn't scale with the player from mid-game to late game. Ways to fix this:
Well this is why the Main Quest creates the Conspiracy Alliance against you. I'm sure you can throw a wrench in the Conspiracy if you know about it, (I really don't like Main Quest - only played it once in totality) but 90% of players are not going to know or prepare that much. So yeah the Conspiracy Alliance can be brutal depending on what the map is like when it goes off. While it can create a "challenge" it's mostly artificial and just devolves into tedium of you forming big armies trying to beat a specific Kingdom into submission and hoping your fellow vassals don't eff up too much on other fronts.
This is a sandbox game. If the game is "too easy" it's because you've min/maxed too many things to your advantage, probably because you've got a couple hundred hours or more under your belt.
Try playing on foot (without RTS Camera), etc. the game can be plenty challenging. I don't think it's in TW interest to cater to what's likely only a few hundred/thousand people at most who the game to be harder. Though TW should do a better job balancing the game overall i.e. so there aren't so many broken perk combos and such, but I digress.
Creating a kingdom and dominating all of Calradia should be more challenging than it is currently. Then when you do it's a more satisfying achievement. In addition it will force the player to have to think more carefully about which attributes and skills to focus on.
Creating your own independent Kingdom is pretty challenging if you ever try to do it legitimately. The problem is most cheese their way to creating a Kingdom - by sniping Towns via Rebellions, saving millions of denars, etc. all while remaining independent. With the A.I. able to team up in sieges now, it's pretty hard to hold a Castle/Town even if you've built up a very strong Party. A Kingdom with multiple Lords/Fiefs should not be negotiating with some "Rogue Baron" anyways who managed to run off with/steal a Castle or two either.
The real issue is there needs to be much better Diplomacy/Kingdom Management. The problem with the late game is less that it isn't challenging, it's mostly how tedious it is since it's really no different than mid-game. All you do is form big armies and fight constant battles without any tangible consequences. Sorry it just gets dull to be sieging Razih for the 20th time in your 10th war with the Aserai.
Also being a Sovereign is pointless. All you do is slap your vassals on the wrists any time they try to declare a stupid war or vote in a bad policy.
So here's what I propose...
1. In order to hold a Castle/Town you must be part of a Kingdom; either an existing one or form your own.
That means if you take a Town after a Rebellion, you have to surrender it to the Kingdom that last held it OR form your own Kingdom. If you don't meet the requirements for forming a Kingdom you automatically surrender it. (I don't think this is a thing currently, but have been gone a few Updates)
If you leave a Kingdom same thing applies. You either give up your Fiefs or you start a civil war with the Kingdom in question when you break away as your "new" Kingdom. (Gee imagine if there was a real point to actually building relations up with other Lords...) And this all brings me to my next point.
*This also mean when a Kingdom loses it's last Fief it's destroyed. However the last ruling Clan can always attempt to re-form Kingdom again if it ever manages to take a Fief. (Ruler clan can buy Fief or convince Rebellion of Fief culture to re-establish)
2. Civil Wars need to be a thing. When Kingdoms reach a certain size (30+ Fiefs) Lords should consider breaking off and forming their own Kingdoms.
Now obviously a Lord with just a single Castle and a tiny clan should never do this, but a Lord with a couple Towns/Castles and big clan - why not? Also when a Lord does break away they should be able to bring some fellow vassals with them to make the whole thing more legitimate (this should be available to player too). Having a big Kingdom effectively break in half will do a lot to make the game more interesting and give struggling Kingdoms a chance to recover as well potentially.
It seems really odd to me that the Calradic Empire eventually got so big that it broke apart, yet this never happens in-game with other Kingdoms no matter how large they get.
Admittedly the mechanics of "Civil Wars" may be difficult to implement. I also very much doubt TW will do this, due to the many technicalities it undoubtedly involves.
I also personally think it should be difficult, if not impossible to conquer the whole map. Maintaining a Kingdom should always be difficult, there should never be a point where you effectively go on autopilot. And no one Kingdom should rule all Calradia. I think the only way to hold a large continent Empire together is a combination of clever vassal assignment (lots of low tier clans) and maxed out Charm.
3. Alliances and actual ceasefires need to be a thing
The constant on and off wars gets really annoying, even calling them wars feels a bit misleading, since it's more just conflict. Smaller and weaker Kingdoms should be able to join up together against a common enemy, obviously this is somewhat possible to do since it already sort of exists in game with main quest.
Also just for the sanity of the player it'd be nice to know how long actual peace will last.
4. Wars need to have some kind of objective/goal
Maybe the point of a war is simply to weaken a rival and inflict causalities on them. Maybe it's to take/reclaim a particular Town/Castle. Or maybe it's simply to extort tribute or stop paying tribute. War should not just be some random event though.
Maybe introduce some "War Fervor" mechanic, max 100 kicks off war declaration, -100 minimum kicks off peace talks. (Nevermind I think this already sort of exists as is.)
i.e. War Fervor builds up slowly for any neighboring Kingdom, every day at peace adds +1, every day paying tribute adds +1x depending on total amount (5000 denars would mean +5 a day)
While at war War Fervor declines by -1 a day (nobody likes long wars), declines by -1x for every Lord captured, declines by -1x for every 1000 casualties suffered. But War Fervor increases if the war is going well +1x for every 1000 causalities inflicted or Lord captured.
War Fervor drops by -100 for all neighboring Kingdoms any time a Kingdom has war declared against it. War Fervor should also drop substantially when a War Objective is met.
I think War Fervor could help with Alliance mechanics. If two Kingdoms are at war with a neighbor, both are losing (- War Fervor), and are not at war with each other - they should form an Alliance against their enemy. Forming an Alliance adds +100 to War Fervor against current target Kingdom. The Alliance also effectively freezes War Fervor between the two allied Kingdoms. However the Alliance breaks as soon as one allied Kingdom surrenders/makes peace with target Kingdom. So they wouldn't be very common or last too long in most situations.
I think War Fervor could also create more logical Tribute. Say every War Fervor point difference equals 50 denars in Tribute. So max is 10,000 denars a day if Kingdom surrenders at -100 and opposing Kingdom is still at 100.
5. Limit max denars for Clan based on Clan rank
Not sure if this is really necessary, but may help curtail some power gaming and give new players realistic goals.
Clan Tier 0: Max Denars 250,000
Clan Tier 1: Max Denars 500,000
Clan Tier 2: Max Denars 1,000,000
Clan Tier 3: Max Denars 2,000,000
Clan Tier 4: Max Denars 4,000,000
Clan Tier 5: Max Denars 8,000,000
Clan Tier 6: Max Denars 16,000,000
6. Kingdom Sovereigns should be able assign targets for Armies
Pretty simple, when at war you should be able to direct an army to siege a particular Town/Castle or raid certain villages. Setting a target costs influence 25 for raiding Village, 50 for besieging a Castle, 100 for besieging a Town. If the army leader completes task they get influence you spent as reward. Could be good when applied to the player as well when they join a Kingdom, a nice influence boot when starting out.
Also would help wars feel less random. I feel this would be most helpful to the player since you can really steer your Kingdom in terms of war without having to lead Armies constantly.