Since it's "now or never" for new features I've decided to throw together a few feature suggestions before we reach the final stretch.

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1. Add Animalistic Trait to various monsters

Reason(s) for Feature: Deal with Bears slaughtering whole villages

My first suggestion is a minor one. Certain creatures in the world (mostly animals) would probably not go around destroying shrines and cities whenever they run into them. Rather, these creatures would maybe kill a few people and then wander off. To address this I suggest adding a trait which specifically prevents their units from destroying constructions.

2. Tile 'Wildness' Rating and Late Game Spawning

Overview: A value on each tile which is used to determine monster spawning.

Reason(s) for Feature: Address lack of late game neutral conflict, reduce the easiness of settlement phase, establish a more apparent distinction in the conflict between civilization and the wilderness. Possible solution to rapid expansion.

My second suggestion is for every tile to gain a new value possibility called 'Wildness'. The values possible range from 0 to 100+, and value-linked descriptive ratings between None and Maximum (I suggest the descriptor 'Savage' instead though, has a better feel to it) , with further value only making it harder to reduce the rating of the tile. This attribute would primarily be produced by actual Wildlands (and lairs) but at the start of the game every tile should have at least a Medium rating in it. Over time while in the presence or an outpost or a settlement's Zone of Control, the tile's value would decrease. The rate of decrease depends on the source of the zone and high-level settlements (especially towns) would decrease it far more rapidly than outposts.  Wildlands and existing monster lairs would increase Wildness values in a medium-large radius to counteract the civilizing influence, although non-Wildland sources can at best combat outposts and small towns.

The only way a Wildness rating can reach the highest rating/values is if it's adjacent to a Wildlands. The only way a Wildness rating can reach 'Very High' is by being adjacent to a Lair or near (within 3-5 tiles) a Wildlands. If the maximum drops, so too does the current value to whichever rating is its highest possible.

So what does Wildness do? New roaming monster units would periodically spawn from locations with higher than 'None' Wildness at a frequency determined by the rating. Their level and strength depends largely on the surrounding terrain or existing monster lairs as well as both the Wildness rating itselt and number of turns in the game. These monsters can be assumed to have always existed but have remained in hiding instead of becoming an active nuisance like their map-counterparts (until they spawn that is). They will immediately do either one of two things: Begin hunting nearby or creating a lair. All monsters will eventually settle down but when they first appear they can be rather adventerous, and thus dangerous. Unlike Lairs, the creatures which spawn from a Wild zone would be of a range rather than set, and thus reintroduce more variety regularly until they're all finally conquered.

Additionally, settling territory in locations of 'High' or more would be impossible, and creating outposts would be impossible at 'Very High' or more. To first tame the lands requires killing all monster lairs and building an outpost. This should slow down expansion but especially that which is done far from existing civilization.

All in all this feature would provide a long-term (re)source of monsters with which to fight using either the sovereign or champions. It also leads to more defense being needed in towns as a nearby monster spawn is still possible until many many turns after its construction, but with a balanced spread in their power depending on how close they are to the civilized zones.

3. Sieges and City Defense

Overview: Require Sieges to take cities and provide benefits to defenders

Reason(s) for Feature: Slow down conquest, enhance the defense of settlements, make attacking a settlement feel more like.. attacking a settlement.

My third suggestion is to implement some kind of Siege mechanic, even if it doesn't necessarily include walls or other terrain features (although those would be preferred). Every Settlement would need to have a Siege layed to it with a minimum time of 1 turn/season per Level of the City (double if a Fortress type). This siege would be interrupted if the attacker canceled the siege action but not if they were engaged in combat and won. Every unit of Catapault present would reduce the Siege timer by -1 turn/season, so a first level city could immediately be attacked if one Catapault was in the attacking stack. If multiple stacks attack the same city they 'join in' as a queue until the city has been taken.
Additionally, each City would gain defenses with which its garrison may protect themselves. If actual obstacles such as walls and gates are out of the question I'll suggest the next best thing: enhancement tiles. Defending units would spawn on tiles already modified by benefits such as 'Fortify' (as used by the Krax) and various traps or movement reducers can be placed across the field. It's a bit gamey, but the beneficial modifiers would only apply to allies while the negatives would apply to the enemy. The AI would need to remember to hold the bonus tiles unless its ranged was significantly worse than the enemy's, but otherwise this shouldn't be too hard to implement. But as I said, ideally we'd see actual obstacles which can be destroyed by catapaults and manned by units. I believe going the extra mile by implementing those would be preferable to this suggestion but I'm just throwing out this one incase that's ruled against.

4. Abstract Trade Routes

Overview: Remove caravan map objects (horses) but let trade routes be seen (if turned on) and directly attacked by enemies or monsters.

Reason(s) for Feature: Horse carts are annoying and don't really work for banditry.

My fourth suggestion is to do away with the caravan units. Rather, I believe it'd be better to use a sort of abstract 'Trade Route' in a manner similar to Total War: Empires. The way these worked was that they established a connection between cities and then nations hostile to one of the parties trading could attack them, raiding all Trade from that faction and granting it to their own. These routes should actively be raided by certain monsters (I'm looking at you, bandits) and by hostile enemies who might want to hurt you but can't go after you directly due to power disparity. Sea trade could even be done this way when that stuff finally makes it into this game.

I feel this method is better as it doesn't have to keep track of various moving units who often get in the way of your roads and disrupt movement. It also provides a clearer target for would-be attackers and is easier for AI to go after it directly.

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I'll add any more suggestions to this thread if I think of them before feature lock down hits.