So I have one map that I created about a month ago. I used pieces on Ravine and searched through the entire selection of pieces in order to make the map. I'm now seeing maps made with pieces that I have never seen. I'm wondering If I either A: Missed them before, B: Pieces where added by 343 or C: someone created some cool custom pieces. The recent foundation map released on Ravine would be a nice example.
No pieces have been added to the game, nor are custom pieces possible. You may have either just not seen these parts or you are actually seeing a combination of several parts and perceiving them as 1 part.
and they never will add new forge palette or pieces to existing maps, they would only on a new map from DLC in the future.
all the dominion and extraction pieces were in the game from launch. before the game was released they took maps and extraction gametype out and decided to sell them seperate (well Microsoft I am sure made the order)... but the extraction pieces in forge were left in.