Even several strats later, some sort of mercenary mechanic on the map still sounds relevant and much-needed. Strategus can seem daunting for people who haven't played it much, there's a lot to learn and it can be unforgiving, that at least was my experience when new on Strat 4. And although i approve of the tweaks that have been made so far and can't wait to see Strat 5, i'm damn glad that it isn't going to be my first strat, Strat 4 was hard enough to learn, Strat 5 has all those mechanics + extra unforgiving micromanagement.
I think just putting all Strategus related forums in one spot, maybe integrating them into the website and having a jobs/quests/mercenary hiring type board and buying/selling board close to the strat interface where everyone could see them would help solve that issue. Currently everything is all spread out on the forum so doesn't get as much use as it could, especially the strat buying/selling boards since they're buried in another section. Put all the boards in one spot with a detailed up-to-date how-to guide, problem solved.
People will create large factions no matter what you do, the question is will it ruin game for others or not.
My suggestion: make towns uncapturable, but lootable.
This way all neutral players and small factions will always have place to build up, there will be trade routes, neutral players can run caravans without stealing anyone's sd(because it doesn't belong to anyone).
We'll still see some town's attacks for profit(since good gear will be very expensive - troops won't be always more important than gold like it was this strat).
How much will attacker get if he wins.. I'd say neutral towns should have tax rate around 15% and taxes should be added to treasury. When someone successfully loot town - he gets all. This way sooner or later it will be profitable to attack a town and it won't be exploit like attack it every week since treasury won't fill up that fast.
I think it might be interesting to have some permanent AI fiefs spread around the map, dunno about all, maybe just a few locations like Dhirm or some of those castles with buggy maps or wooden walls that can't be destroyed. Like he mentions, it'd give people not in clans fiefs to use, also maybe help bring bandits back into the game too. Trading was much more fun early in Strat v3 when there were mostly AI fiefs so clanless players could go anywhere and you never knew where bandits might be lurking.