I believe that I will start construction on a moat this coming weekend
Nah, don't need a moat. They have already demonstrated that zombies can't climb. I think a nice wall would be effective enough.
I suspect that Hershel is going to go the rest of the way around the bend now. I don't know how it will happen, but I expect Hershel to do something really really crazy now and end up as zombie food or perhaps show up as a walker himself. But it will be something that renders the farm too dangerous to stay there.
I also expect Maggie and maybe the young guy to join the group. Maybe even Otis' wife will join the group, which will add fuel for conflict when what Shane did to Otis comes more into the light.
I figure they will keep several of the new characters in the story for awhile, simply so they have plenty of people to get eaten during future zombie attacks.
Also, from a production stand point, keeping a steady flow of new characters that they can build up if they want to will help to keep the actors in check when contracts are negotiated. The Walking Dead comic books are known for the authors willingness to kill off pretty much any character. The show producers just need to keep that alive so the actors don't get comfortable enough to start asking for huge amounts of money.