Sea travel - cheaper but less certain.
As a knight I pay like 35 gp to procure a sea transport for my entire company with carts and siege engines. As a lone adventurer I pay like 15 gp to board a ship. This doesn't sound right.
I suggest lowering the price significantly. Several gp should be enough.
But a certain twist can be added - while nobles can just rent a ship and sail whenever they want, commoners can on only afford to hop on a ship already bound for some port. So every coastal province could have 0-3 ships (more ships in more developed regions) leaving to random ports, destinations changing every day.
Interesting idea. Might work.
More consistent adventuring patterns.
If there was a large battle with lots of casualties or starvation, a region gets a significant increase in undead activity
This already happens.
and higher bounties.
This is dependent entirely on the Lord. Bounties don't come out of nowhere; they have to be explicitly set.
Provinces where infrastructure is undeveloped or have severely deteriorated might experience an influx of monsters.
This way we'll just have places where adventurers can expect to find more adventure, thus congregating there and joining forces.
Interesting idea. Might be worth looking into.
More adventurer/noble interaction in intrigue. More adventurer/adventurer interaction in PvP
I don't buy noble assassins. Leave spying and assassination to adventurers!
Adventurers need to be able to gather more information (especially military info) from the regions they're in.
Give adventurers increased skills in stealth. Make it very hard for nobles to spot and catch an adventurer. Make it fairly easy for another adventurer to track a colleague.
So that to weed infiltrators from their lands, nobles would have to employ other adventurers.
Sorry, this is the kind of thing that's not going to happen.
Adventurers are not going to become just another tool for nobles to use to "win." The adventurer game is
deliberately mostly separate from the noble game.
I do have some (vague, so far) ideas for improving the advy game, though.