I've given it some thought, and here are some of the things I've come up with:
Salamandastron has become a large coastal city and capital of the Mossflower region. The top portion of the mountain has become a dock for the many airships that move in and out of the city. The Long Patrol are kept busy patrolling the city and the countryside. The city is governed by the badgerlord and a council of representatives.
Redwall abbey has changed very little. The most that happened was a small farming village forming near the abbey.
As for the story, I have two ideas. First, the location of several artifacts have been discovered and the badgerlord has put up a reward for each one found. Second idea, a strange plague has struck salamandastron and several ships have been sent out to try and find a cure.
The technology is approximately mid 1800s era. Just with a lot of steampunk/clockwork tech. I am also debating having some kind of magic, but I'm not sure.