One thing I highly recommend is to look up and find all the Fell Arms. If the final dungeon is available, they should be as well.
You wont get to fight the full final boss without them. And the game doesn't mention them much at all.
Depending on you far along you are, there should also be a new area in the North West of the map that you can access now. If you can access the final dungeon, you can go there. It's not necessary and has no bearing on the plot, but it's a great place to go. There are a few good weapons and some factories you can get. And you can get outfits for almost everyone there. Theres a stage where everyone (except Patti and Rita) can get some. Theres also a test Rita can take that will get you a damn cool outfit for her. I HIGHLY recommend looking up the answers, because its random trivia about Namco Bandai games that not many people would know off the top of their heads.