I love this game! I'm so glad the new version is out!
I found some secret items in the new areas, and figured I'd share. See the attached images for visual reference (no image for (2)).
(1) In the first new area (forest), in sort of the center-rightish part of the map, there's a room with a sign pointing to a chest. You have to roll over an invisible hole in the floor to get to the chest, but it's empty! But! If you climb into the wall behind the chest, there's another chest with the ultimate greatsword inside! It lowers your defense and movement speed, but it's still awesome.
(2) This isn't very secret, but in the upper-left of the forest, there's a difficult Gray Knight enemy. If you defeat him (took me a few tries in Hard mode

), you get access to two treasure chests. One has an item that increases defense (I may not be remembering this correctly), and the other has shoes that nullify fall damage. These are both equippable with any other items, so they're pretty great!
(3) In the second new area (castle), just outside the upper-right save point (or silent empty room for those in hard mode), there's a secret passage. To get there, you have to run off the ledge leading to the save point at just the right speed. This is trivialized if you're wielding the greatsword I mentioned in (1) above, since your maximum run speed is perfect. Otherwise, you just have to play around with different speeds until it works. It helps to have the shoes from (2) equipped. Beyond this secret passage are the ultimate Attack and Stamina equipment items.
(4) Still in the castle area, just up and to the right of the save point from (3), there's a block puzzle room. If you push one of the blocks to the bottom right, you can gain access to another secret passage (there's a slight indentation visible in the wall). This leads to the Old Key, which unlocks the door in Area 1 behind all the Rusty Key doors. Go there and get the ultimate Knuckle weapon!