I am always interested in Expressions but I have yet to find one I would use because I am doing something similar without one. You could also just give the Object an ID : New, To Be Removed, Existing to Remain... And of course you can use a different pens and backgrounds to graphically differentiate them.
Also, it helps if your using a Legend to make it a Live Legend as a Wall Types Legend, Electrical Legend... it keeps everything as parametric as possible and the Live Legends are much easier to read and find what your looking for than tiny little icons and names that are abbreviated by necessity in a Favorites Folder. Just drag a copy from the Legend to were you need it. Pick up properties with the Syringe, Copy & Paste...everything you need is right there where next to your Floor Plan. No need to be hunting things down in folders so much.